You need to have a conversation with your SO !!  They lead the example on how to treat you and If SO treats you like an outsider they are telling  SD it’s ok to as well!! That’s your home as well you deserve to be treated with respect by both of them!! Trust me talk about it now if not things will just get worse!! 

Time for a zero tolerance and bring the pain total confrontation approach to this crap.  

I have only kept one secret from my bride regarding my SS (at his emphatic request) and it came back to bite me in the ass 3yrs later.

You and DH are equity life partners.  There are no secrets.  DH needs to give the very clear message to SD that any discussion they have will be shared with you.

PERIOD!

That was always the case in our marriage.  Except that one thing.  Never again.

DH needs clarity that the sneaky conversations with SD end immediately or they are both out.

The trust and full disclosure between marital partners is sacrosanct IMHO and in my practice of my marriage.  If sneaky stepspawn want to backstab the SParent to the SParent’s spouse  all bets of confidentiality are off.  Outside of ethical restrictions applicable to  some professions confidentiality limits completely include full disclosure and discussion with a spouse.

At least that is how it works for me in our marriage.

As I have shared elsewhere in this thread, I did keep a SKid related secret from my bride once.... for three years.  I am now able to share that confidence.

When he was 17 SS was restricted from being on his laptop except for school work.  He was also not allowed to have his BR door closed.

We had just disenrolled him from Military Boarding School due to failure to perform facilitated by the SpermIdiot helping him hack the school firewall and the two of them gaming (WoW) all night so SS was comatose in class every day.  He failed the only class that  semester that he needed to graduate.

So at mid year we jerked his ass back to our home, enrolled him in our local HS (the best in the state) and told him to graduate on time or he would be dropped off at the Philly homeless camp on graduation day.  He knew no one as we had moved a few weeks after he started at military school.

We scared the shit out of the kid.

Long story short .  His door was closed, I tapped on the door, walked in and  caught him surfing gay porn.  He was adamant that he was looking at non gay porn and had clicked the wrong thing.  I told him that if he needed to talk with his mom and I about anything that we were there for him and his biggest fans.  He said that there was nothing to talk about and begged me not to tell his mom about the porn.  I kept his confidence.

3 years later he came out to his mom and I.  We were living in Qatar at the time and he was stationed in Texas.  The call had a different vibe than our usual family calls.  He was all over the conversational map, was stammering and obviously nervous.  I finally just told him to spit it out.  My bride was confused and was looking at me questioningly.  I whispered to her “Honey, he is trying to tell us that he is gay”. As soon as those words left my lips he told us.  

His mom was floored and had no idea. I had suspected for years.

We had a great talk.  I told him that I loved him and was proud of him.  I then I told him “Son, life can be challenging and this will make it more challenging.  Some people will take exception to you being a gay man.  Keep your head in the game and be safe.”

I was happy that he had worked through his recognition of his sexual orientation and would be able to have what his mom and I have together rather than being a sexually amorphous recluse. He had a ton of GFs in HS.  There was just something off with him in those relationships.  His friends, both male and female, gave him crap for having no sex drive.  The young women were serving it up on a silver platter and he was not playing ball.

The poor kid struggled with thinking that there was something wrong with him and was in a bad place for a few years.  I wish he had come to his mom or I for help.

So, during the phone call the little little shit threw me under the bus.  Through group tears my wife was lamenting that he hadn’t told us and we would have wanted to be there for him and to help.  Then he said “Mom! The moms always know and besides dad told you. You guys talk about everything.”

She looked at me with mom vengeance lightning bolts shooting out of her eyes.  I said “Thanks for throwing me under the bus son.”  He and I went on to tell her of the porn incident during which I reminded him that he had not come out to me and had expressly implored they I not tell his mom about the porn.

And that is the one time in our marriage that I have kept anything in confidence  from my bride...... other than when I have shopped for gifts for her.  I don’t  t tell  about that.  Sneaky guy that I am.

“It seems to be most of us are burdened by being privy to way more information than we need or want to know.”

Absolutely.  You nailed it.

Unfortunately so much of blended family life is like watching a slow motion train wreck unfold that a person just can’t stop watching (listening). 

Let SO take care of the lizard. That's an easy one.

The conversations, though...that's tough. My SD does not live with me, and it seems my husband NEVER talks to her because it never happens in our house. But that's because the calls always happen when he's in the garden, on his way to work, or elsewhere. It is an odd feeling that he can't even make a call in our house to her. It makes me wonder what the hell is being discussed that I can't hear half the conversation. Whatever. I've given up on that situation.

But in your house...that's even more weird. When SO is giving you said crap, have you ever mentioned that you are not part of 90% of the conversations? What's he say about it?

I'm sure if most of us took a few seconds to remember middle school and the mean girls, you would know exactly what this is.  Relational aggression.  Its the evil triangle of middle school where 2 teamed up to make the third feel like an outcast, unwanted, not good enough to join the group.  Its one thing to experience this at school,  Its a totally different thing for it to happen in your home with your spouse and family.  

That sounds absolutely horrible. Thankfully you saw the whole thing for what it was and took action.

Glad you got to enjoy the rest of your vacation.

this can be a very excluding behavior on both of their parts. Your dh can't have it both ways. He can't expect you to be the other adult decision maker in the house but then not include you in conversations. Personally, if it was me I would call out HIS behavior first and tell him how it makes you feel.