NC for this as quite outing. Growing up my "D"F was pretty emotionally abusive to my DM, my DSIS and I. DM eventually managed to get the strength to leave him when we were teenagers, helped in no small way by one of her very good friends who treated us as family.
To cut a long story short, five years later it turned out my father was in a relationship with this woman. When my mother confronted her about it she said that DM had lied about DF's behaviour. Turned out all the while she had been pretending to be DM's friend she had been in contact with my father behind her back. I know maybe it won't sound like much to many on here perhaps but it was just such a betrayal. My father's abuse is incredibly subtle - think gaslighting and emotional manipulation rather than shouting/screaming/physical violence. This woman had talked to us all extensively about what he had been like to live with and then to just go and dismiss it all - just, I felt I could never forgive her or him. I didn't speak to him for a few years after that happened and I haven't seen her at all since.
I eventually regained some sort of semblance of a relationship with my father , it's not great but it's ok. I am now engaged and getting married in the Spring. I have chosen not to invite my father's partner to the wedding. He has made me feel incredibly guilty about this. I have told him I understand if he feels he can't attend himself but he says he "has" to as he is my father. I don't really want him there either if I'm honest but I know that would cause family ructions on his side that I just don't have the emotional strength to deal with.
Anyway, now he has found out that I am inviting my mother's husband and all hell has broken loose. Like, of course I am going to invite my stepfather. He is an absolutely lovely man, treats my mother like a Queen, has worked hard to build a relationship with my sister and I, is a wonderful father to his own three children (who I get along well with and will also be there).
I am now feeling massively pressured that I need to invite my father's partner too (as far as I know they aren't married but with my Dad you never know, he never tells you anything - he was married twice before my my mum but she only found that out when they were getting divorced). I just don't want to see her.