Hi everyone, I thankfully haven't needed this board much lately

I have a dilemma about setting boundaries, which is very hard for me. I never know when I'm being unreasonable (as in my uBPDmom's eyes, most boundaries are totally unreasonable). But this is a boundary I'm trying to set with my dad, who is not BPD.
My parents separated when I was 4, and since then my dad has been with his second wife (I'm 36). I never felt she accepted me as part of the family, and there was of course A LOT of friction during my parents separation due to my mom's BPD that my stepmother also had to endure. I've always felt that when she looks at me she sees my mother, and has treated me accordingly. I've never been close with her or really felt any emotional attachment with her, and have many times felt like I was treated as a second class family member (she has her own kids and my brother and I would visit on weekends which I"m sure contributed to the "outsider" feeling). Often I've felt that she resented his children and expected him to give us up and be a family with her and her kids.
When I became an adult, I wanted to put the past in the past and just have a normal, peaceful relationship with her. But there was always some issue my dad would bring up to me - "why didn't you call stepmom back? Why didn't you answer her last email?". This would go to the extreme, like she had emailed me two hours prior and I hadn't checked my email yet, but she always just "knew" I was being passive aggressive by not answering. My dad never seemed to see the ridiculousness of this kind of thing, she was always very paranoid about her and my relationship. In truth, she would do very passive-aggressive things like rearrange their wedding date to a time she knew I was out of town. Which I was expected to say nothing about, because nothing is worse in our family than making waves by speaking the truth. I feel I've always been expected to conform around her, and keep quiet if I was unhappy with something.
There have just been so many ongoing issues as an adult and I feel like at what point can I stop hitting the reset button? The straw that broke the camel's back though was when my niece was taken into custody by CPS and could not stay with her parents for a time until things were worked out, my dad/stepmom and I were sharing duties taking care of her. She was 1 1/2 at the time and everything was stressful and chaotic. One day I get a text from my stepmother that her and my dad can no longer help take care of my niece because it's just too much. I asked my dad about it and he said I can just give her to CPS if I didn't want to do it by myself. I was horrified, devastated, as to me this is not how family acts. I didn't understand how they could be so cold towards their granddaughter. A couple of weeks before my dad had been talking about going into early retirement to take care of her, so that gives me an idea exactly whose plan this whole giving her up thing was. I didn't speak to them for over a year because of this.
After I managed to reconcile with my dad, he wanted to surprise my stepmother by bringing my niece to see her. I should have known this was a bad plan, as she must have been blindsided. HOWEVER I don't think that excuses her behavior. She would not look at or speak to me - OR MY NIECE, a 2 1/2 year old child. At this point, I was done trying to have a relationship with her. My dad had said she really wanted to see the child, but her actions told me otherwise. I felt she saw my niece the way she saw me and my brother, as a second class family member. To this day they have no relationship with their granddaughter, but they are fully involved in my stepsister's new baby. I don't want my niece to wonder why she is treated differently than her stepcousin, it makes me sick. One day she is going to ask why she never saw her grandpa.
Fast forward to today, and I'd just like to avoid my stepmother. We just have such vastly different philosophies on life and family, what is acceptable behavior and what is not. I can forgive my dad for abandoning my niece (and yes it hurt particularly bad because it echoed the emotional abandonment I'd always felt from him in regards to my stepmother). I don't know what they talk about between closed doors, but I don't feel he's ever told her she has not treated me right - like I doubt he said anything about her ignoring me that day. So I'd like to have a relationship with him but not with her. But lately he's making our relationship conditional on me having one with her as well. He knows how hurt I've been and that there's been no resolution, but he keeps inviting her places. I've told him that I don't care to interact with her anymore than is necessary, I don't hate her, I just don't see us having any type of relationship. I was supposed to go to lunch with my dad today and 2 hours prior he tells me that he's bringing my stepmother. He knows exactly how I feel as a similar incident happened a few weeks ago, and I told him my boundary - I don't want to socialize or spend any unnecessary time with her. It just opens the door to her paranoia, demands, and expectations and I can't deal with that anymore. So I told him today I can't come if she's going to be there. I was accused of choosing to act like uBPDmom, which I find ironic because I feel the whole issue is that stepmom seems unable to view me as a separate person from uBPDmom. My dad used to see me without telling her (I assume because she'd give him a hard time about it) but he's had a stroke recently. He's been very dependent on her since and I think he thinks demanding I force a relationship is him being loyal to her. Or he's afraid that she will not take care of him anymore if she finds out he's seen me without her. It's just all very bizarre. I tried to help him after the stroke but after I said I didn't want to go to Ikea with stepmom all my offers of help have been refused.
I've been told I am causing friction in the family which is detrimental to my dad's condition. I feel it's always been very easy for them to blame me and expect me to change. I was always the obedient, quiet, non-demanding, self-sufficient kid in the family. I told them I agree that this is not good for my dad's health, but I'm not the one making unreasonable demands. I feel I'm expected to ignore my own feelings for the sake of others, as always (my mom & stepdad expected me to do the same).
Is my father being unreasonable expecting me to have a relationship with stepmom to be able to have one with him? Am I being unreasonable expecting him to see me without her present?
What would you all do?