Most young kids ARE self-centered ... especially if they have Disneyland bio parents. 
 

She probably senses the tension between you and her, knows that a new baby might take the attention from her, and ramping up the convos about herself so she doesn't feel left out and pushed to the side when the new baby comes. 

ALOT of kids are like this. They panic when a new kid comes along bc they don't know what to expect and some of them are fearful about what will happen to them during this process  

Beat her to the punch. Take her to the bookstore or library and get her some sibling type activity books and reassure her she will always be important too. 
 

Be proactive. 
 

Print some big sis t-shirts and let her proudly wear them. 
Take some family photos of her holding the new baby ultra sound. 
Let her help pick out the baby decor, etc
Let he be as involved (as you feel comfortable doing) which may ease some of her anxiety and also lets her think she's running the show but really you are when you are being proactive with it. 
 

Also, have more baby celebrating with your friends, family, husband when she is with her mother so less you have to worry about her responses driving the conversation where you don't want it to go. 

Honestly, most of what she said sounds pretty typical for a pre-teen. Most kids that age are pretty self centered. How did you want her to respond?

I endure it constantly and it drives me insane. I can't even tell you why & most of the time I am not even trying to have a moment, just a general conversation. To me, it really has more to do with these kids not understanding or learning how to be part of a conversation in general because I don't recall my kids ever doing this but my SS does it constantly. If we discuss anyone other than him he has to do almost exactly what you describe-asking questions about what HE did at that age or if we tell a story about John he immediately tries to follow with...I once did...and tell a similar story (which half the time makes no sense).

Like seriously enough already. Sometimes I just want to shout: will you just stop talking and listen for once? Or...Nobody cares right now! Yeah I sound mean but damn it gets OLD. The thing is these kids get plenty of attention & I have noticed at least here, the more SS gets the worse he is, not better. 

Lather..... rinse.... repeat.

Do not tolerate the "look at me" crap.  

"This conversation is about our baby and your younget sibling. This convesation is not about you. or your mother.  Now, participate respectfully or you will not say another word."

If y ou really want to shut her down .... "You are not even my favorite kid in my marriage and right now you are the only kid in my marriage. You should work on not being an invasive entitled brat."

I went through a variation of that when I was pregnant with Dd8. And I'm not sure if it's an only child thing, or just stepland norms, that these children have insecure attachments with their fathers and it plays out in their behavior. 

I made sure I told the boys that DD8 would not replace them to their dad. That they all would have a special place in their dad's heart. I still was highly annoyed with both for different reasons but I tried not to show it. SS14 was the one always competing for attention. "If daddy gives the baby money to get her hair done, then he has to give me money to get mine done" out of the mouth of then SS14. I cringed inside when he would touch my stomach because I knew it was a show, he wanted the good boy image to his dad. What confirmed it for me was one day when then ss9  me that when ss14 found out I was pregnant he was kicking and punching the walls (meanwhile he acted happy when we told him). 
When I was 7 months pregnant ss14 had turned 15 already, and asked what would happen if he punched mE in the stomach. According to DH, he didn't really mean anything by it, he was just trying to be funny. I made sure I kept one eye open around him. 
I remember not wanting him specifically to hold DD. I thought he was malicious enough to drop her and act like it was an accident. DH and I fought a lot, he wouldn't believe it. I blogged about all of these things. 
At the end, what I can share with you is that it may be in your child's best interest to have a great relationships with their siblings. It sounds like your DH is very involved in SD12's life, so helping her to adjust will benefit you via your new baby. Having her want to help, not feel competitive and feel included will take work, and sometimes be annoying, but it's worth it. 
Regardless of how the relationships with SSs and I turned out, I don't doubt for a second that they love Dd8. They won't know if, but it was the work on my part. 
I do think if you can have SD12 pick some stuff out for baby, or help organize baby's toys/clothes it may let her feel more included and less likely to feel replaced (SS18 then ss9 picked and organized clothes). 
I do think your DH has to do the heavy lifting on this, and he has to communicate and teach SD12 how to socialize and how to see things from the viewpoints of others. I know a lot of posters are saying "most kids are like that", and that might be true, but it's up to the parents to help that kid see themselves how others might see them (for example, if she can imagine how it would feel like to have a birthday but have a friend talking only about her birthday and comparing it). 

"Comparison is the thief of joy", maybe DH can teach her this too. Your baby's experience in this world will be different than hers. If she wants to know about her mom and how she felt about being pregnant with her, she can ask her. Have DH correct her (SD, if you want to pictures of your mom pregnant, ask her later but right now, we're talking about Rose Pedal and baby). He needs to be honest with her, in private, and teach her that the behavior is annoying, and encourage her not to repeat it. 
Get ready, because when the baby comes, the shitshow really begins for quite a few years. And I just dealt with boys, and they weren't only children. Hoping it's easier for you than it was for me. 
 

IMHO, you don't have discussions in social settings that don't "involve" the parties present.

She was participating in the conversation that you decided to have when she was present.  She participated with her own perspective and opinions.

You invited her into the conversation... by having it in front of her.  did you expect her to sit like a dumb mute while you wax on about your belly and baby names?

She is showing interest in her new sibling.. to be honest.. that's a positive.  Because she didn't understand that you need it to be about "you you you".. she is in the wrong.  

Don't talk about choosing baby names with someone there that won't and you don't want to have a say in it. 

If you don't want her to talk about her relationship with your future child.. don't talk about future child with her.

as others said.. teens and she is preteen are self centered.. at least she isn't talking about getting rid of her new sibling... please look for the silver lining. 

Unfortunately.. you are not the first and only person to have a baby.. and not everyone is going to understand that you somehow want them to be simultaneously happy.. yet not involved..especially when it's her immediate relative.. her sibling.

I can completely see how annoying it would be when she turns everything around to be about her.

However, it's not polite to have a conversation in front of people when you don't really want their input. I would find it quite uncomfortable to be in a group of people talking over me and expecting me to stay quiet. Also, it takes maturity to know how to contribute to a conversation without stealing the spotlight. Most ALL people I know will contribute with a little story about their own experiences on a topic, and many haven't learned how to contribute without making it all about them.

She was clumsily trying to be a part of the conversation. If her input was not desired for names, don't talk about naming the baby in front of her, etc.

If you have younger siblings, maybe you can share a story about how you were worried the next cute baby was going to steal the show, but how much you enjoyed having a younger sibling looking up to you. That's one way to reassure her that she will still be important.

Of my 9 gkids, 2 are attention seekers, my GD40 and my stepGD32.  Both have been like this from the beginning.  GD was an only child and only GC on the other side so I put it down to that.  StepGD was intensely jealous of her brother and that seemed to be the reason.

Whatever the cause, these people with the intense self-involbement are so tedious to others.  I hope you can navigate with all the Steptalkers' suggestions.  But, I'd be annoyed, too.

The family is perpetuating this toxic SD obsession.

Thats worrisome and something to discuss with your DH. 

Unfortunately I do not have my own kiddos, but congratulations on your impending bundle, and best wishes for your mom.

I like all the kind suggestions of SM's whove been there done that got the tshirt.

On the bright side... at least she SEEMS excited. For now anyways. When I was pregnant SD12 announced she decided she didn't want a brother anymore. I literally said, "it's not your choice." Her attention seeking behaviors formed into acting like DS didn't exist. Keep her involved until it becomes overbearing or negative. DH gifted SD12 a "big sister" onesie as a way of telling her with a surprise and she immediately burst into tears :/ At that moment we realized this is something SD didn't want ANY part of, so I quit pushing it and have yet to initiate involvment since. 

I can understand how frustrating it is when you are wanting to talk about your baby and SD just keeps wanting to interject and insert BM into the equation.  Next time you might address it head on “Well SD these are great questions for your mother, you should make sure to ask her that.”  Also pre-teens, especially pre-teen girls chatter a lot.  I love my BD17 but she chatters non-stop sometimes and I have to ask her to give me a few minutes.

Hey Rose_Pedal,

I’m not surprised that you’re running out of patience with your attention-seeking SD as you’ve got a helluva lot on your plate! You’d no sooner lost your dad when your mom (whom you adore, as I adored mine) was diagnosed with Stage 4 cancer and, although you’re more than willing to donate the kidney that she needs to survive, doing so will likely prevent you from having more children than the little one you’re now carrying. The stress from also having to wait until your mother is 6 months cancer-free before your kidney can be transplanted must be like waiting for the axe to fall.

And then, there’s the pregnancy itself. Yikes! With body changes and hormones raging (been there and done that 3 times), the normal aggravations of life seem multiplied and you are already carrying a heavy, heavy load.

The other members have made some good suggestions but it seems to me that you need to get your husband completely on board concerning ‘Holly’s’ neurotic need for constant attention and her neglect of school work. In today’s competitive world, higher education is essential and Holly’s poor performance will be a considerable drawback to her future accomplishments.

The best way to get your DH more concerned/involved in raising his daughter to be a functioning member of society is by dressing up your complaints as concern as in “My darling (rather than, "Hey, asshole"), I’m seriously concerned about Holly’s poor grades! What can we do to have her bring them up to standard?” Apply the same procedure to the attention whore behaviour (“How can we convince Holly to be less focused on herself? I’m worried that she’ll be incapable of either making or keeping friends if she takes that attitude into high school!)

Trust me, I know exactly how difficult these entitled ‘daddy’s girls’ can be; my husband’s daughter was a spoiled, lazy, mini-wife extraordinaire. When we opted for family therapy (my idea and it sure helped), the counsellor stated that he’d seldom seen such an immature thirteen-year-old. If my husband had not been so determined to make it work, his daughter would have ended our marriage.

Hang in there, Hon. Things are bound to get better! And BTW, why rose_pedal instead of rose_petal? I'm curious.

I'm sure you will keep a very close eye on SD when in the presence of your child.   These are the types that outwardly express interest and "love" for their half sibling but have actually caused physical harm in some cases due to severe jealousy.   

I think I'd rather have an skid that shows no interest in baby to be and keeps their distance.  The former too easily fools onlookers such as biodad and other family members.

He was clear about that.  He has 3 younger also out of wedlock Spermidiot half spawn by 2 other baby mamas and wanted nothing like that in his real life.  His Spermidiot's  out of wedlock serial statutory rapist breeding shit storm turned SS off regarding the topic of younger sibs.

Not that we would have not had kids of our own if we had chosen to have them. His opinion would have been irrelevant.  As it turned out, SS is an only child in our marriage. His mom nearly died from her pregnancy with SS with severe Toxemia/Preeclampsia so her OBGYNs always advised strongly against any further pregnancies. I would not jeopardize my bride's health and life for Rags spawn.  It never entered my mind.  She wanted more, but fortunately she is brilliant and would not joepardize her life just for another kid.

My thoughts on the opinion of elder sibs, or half sibs, regarding their parents having more kids is that.... could not give a flying rat's ass what a kid thinks about that topic.  These are adult decisions and existing kids suck it up and get no say, or even an opinion IMHO.

For damned sure they do not get to be toxic on the topic.  A kid who would mistreat a younger sib or a SParent would rue the day, and every other day after that, because my ability to tolerate their toxic presence after they pulled that crap would be zero.  There life in my home would be one such escalating abject misery that they would avoid it like the plague.