I don't know how to help my husband, give him the support he needs to handle his daughter while still creating the boundaries I need to raise my 10 year old and have a healthy marriage with my husband/her father. I'm frustrated because I feel like he entitles her to this type of behavior but I also am new to BPD and don't know where the line is drawn between her illness and her spoiled brat behavior.
These are such tough relationships, I'm so sorry you're in this position. It sounds like you are at the end of your rope, and need to focus on your own needs right now. How did your H respond when you said enough is enough, and you cannot be around SD26 anymore?
My SO has flimsy boundaries with his D19 too, and I realized when it comes to her, he basically becomes a sort of vessel for her behavior, bringing those behaviors closer to me. Since I cannot make him have better boundaries, I found it was much healthier if I took charge of my own boundaries. That means that I decide how much time I want to spend with them together -- it's actually easier for me to deal with D19 on my own. Together, they regress, and it is much tougher for me to be around the two of them.
I don't ask permission or engage in explanations for my boundaries, I simply say (gently) things like, "No, you two go have breakfast together. I have plans to do xyz." If SO insists or keeps trying to include me, I repeat what I said, almost exactly. Or I just say, No thanks.
When D19 came to live with us this summer, we ended up seeing a couples counselor together and that helped clear the air a bit. It isn't certain if she will live here next summer again, though I have said if she does, that I will need couples counseling to help me get through it, and fortunately, SO has agreed. Would your husband agree to something like that?
D19 was just here for the holidays and I chose to spend a week with my son for 10 days on our own, overlapping with D19 for a few days before she left to spend time with her mom in another state. It was hard for SO that I left for the holidays, and I feel sad for him that it has come to this, but D19 is so far up in his stuff, and that means my stuff, that it's better for me if I limit how much time we spend together. It actually makes it easier for me to be with her when those times do happen, because I can tap into some compassion and emotional strength that I build in her absence when I take care of myself.
I also learned about BPD skills from this site and from reading, and began to use the skills with D19, and stopped trying to rely on SO for guidance since he is too enmeshed in the relationship dysfunction to see it clearly, which means he cannot solve it effectively. I model the behavior I think he needs to use with D19, and don't engage in triangulation or solving D19's problems, ever. If she is being needy or clingy, I remove myself from the dynamic. I try to not be judgmental and when I need to vent, I do it with a counselor. Once I get out the big feelings, I am usually in a better head space to talk directly to SO.
At the beginning when things first came to a head with D19, I pushed pretty hard against SO to change overnight and you can guess how well that worked out. Now, I try to be non-judgmental and focus on my boundaries and doing what fills my cup. If SO does things for D19 that seem enabling, I allow myself to say once, "Maybe doing the same thing over and over again isn't working. What if D19 had the opportunity to learn how to problem-solve on her own?"
I think codependent people have a hard time experiencing their own challenging emotions, so they rescue/fix/save others to protect them from similarly challenging feelings. When the truth is that emotional resilience means experiencing them, and having the satisfaction of making it through them ok. I try to validate that kind of thing with SO, and have compassion for him, without enabling him myself. He has a very difficult, challenging daughter and not a whole lot of skills to break out of the dysfunction. I cannot make it better for him until he is motivated to do that on his own. It's basically about applying to him the same thing I think he needs to apply to D19.