This is a big pet peeve of we SM's on here: Mini-wife syndrome and Electra-complex going wrong. DH is to blame. Try to find some blogs on here that speak to those two things. Been there; done that; feel your pain! Mine is turning around for the better…slowly…ever so slowly, but I see the light at the end of the tunnel--hope you will too! It took a lot of work and nagging and fighting on my part... and the turnaround was from SD's therapists to convince him that the skid was out of line trying to be his mini-wife, and BFs don't have the sense to know what is going on.

Maybe I'm just being emotional but this made me cry a little....
You need to talk to your H about this and tell him how you feel, and hopefully he will understand and try and make you feel like you used to.
I just don't know what to say. It must be very hard to feel like the third wheel. I felt like that a little at the beginning of our relationship but I decided that I wasn't going to take a back seat to a 10 year old and I kinda butted my way in. If he was sitting on the couch with SD I would sit in between them and cuddle up to SO. I would decide what we were going to do that day and ask SD if she would like to join us. You need to establish yourself as the woman of the house. She probably senses that it upsets you and she's doing it to try to get to you and see how far she can push it. If they are in the corner telling secrets, speak up and tell them its very rude and they are acting like children. Make yourself big and vocal. Don't sit back and watch.
If you don't put an end to this its only going to get worse the older she gets. This kid needs to understand that he place in the family is to be a CHILD. Not a wife. You are the wife.

Living the life right now. DH and I were best friends, have been together for almost 7 years, never fought about anything (other than BM stupid shit) UNTIL SD16 moved in last year. Within one week of her moving in we had a fight that was bigger than any fight I have ever been in with ANYONE... and guess who spearheaded that fight? You got it... SD16. Granted DH was to blame too, because he chose to side with his NEW wife and jump my shit. SD16 has been living with us for almost a year now and there are days that I don't think DH and I are going to make it. I hate this girl, I really do. Same as you, she and DH are like bestest buddies and BS4 and I are chopped liver now. The worst part is that they were not even close before she moved in. Hell, DH saw her ONCE in the first 4 years of her life! He always used to bitch about how manipulative she was, how she only called him when she wanted something, etc. But the minute she moved in, all of that was forgotten and she was his new wife, the Perfect Precious Princess.

I am so sick of waiting for any little scrap of attention I can get from DH when he is done taxiing, catering to and revolving his world around SD. The worst part is that when DH TOLD me that SD was moving in (yes TOLD me, it was already a done deal) I cried and cried and DH was like "why are you crying" and I told him... because this is probably going to be the end of our marriage. Of course his answer is always.. "just hang in there, it's only for a couple of years" Whatever. She is a spoiled, entitled Perfect Precious Princess who is not going anywhere. She'll be living with us or off of us for MANY years to come.. guaran-fucking-teed.

I live for the weekends that she goes and stays with a friend. It's the only time I am ever happy, because I have my husband and best friend back. when she is not around, then BS4 and I are worthy of his attention and time.

"One of these days I'm going to leave. Eventually the mere thought will no longer break my heart."

I'm working on getting there myself.

The mini-wife in my "marriage" is 16. They do not outgrow the behavior, unfortunately. You should make your future plans with that in mind.

At 16 she should be getting more into her friends, even boyfriends. I saw a change with my SO's daughter when she started driving and could get herself around. Unfortunately she didn't date until she went to college. She is now almost 20 and in second year of college and it's funny now because she is conflicted with how to react to things. She is home from college with her BF, but still wants to be Daddy's little girl, still wants 'first bill' in his life, and when he pays attention to me instead of her...she makes a pouty little face, then kinda remembers that she is with her BF. I much prefer to see his 2 girls when they are with their BF's, their BF's are very nice to me and that makes the girls nicer to me because God forbid they wouldn't want to look bad in front of the BF's. So when they get a social life it definitely helps. Hopefully your SD will get a life of her own soon.