My stepdaughters found about their father's affair with a married woman with 2 dd of her own. At this point, all four of the daughters on both sides have stopped talking to the cheating parents.
For 2 years now he has tried to force his adult children to accept his new girlfriend and forbidden them from talking to me at 28 and 21 years of age....
Youngest daughter completely blanks him and hasn't spoken to him in 2 years. Oldest daughter was so ashamed of him that at 27 years of age, changed her last name to her mother's maiden name as she says he is dead to her.
If he hadn't gone above and beyond to force this new blended happy family I am sure the girls would have in time accepted her as they did me.
The first time the oldest stepdaughter met me even though her parents had been divorced for years before I met him was off to a bad start. I said hello and she said "Fuck off" :)
Today we are even closer now than ever and I still do not say a bad word about her father and talk to her about in the future having a relationship with them. She says never as they both cheated on their spouses and wants nothing to do with people who go through life hurting those they supposedly love.
I'm biased of course but these kids need time to process all the life changes and no matter how old they are or how much you push or try to force things you will only push them further away.
I won my stepdaughters over in time by never commenting anything against their mother, being open to listening and never trying to replace their mother. I always encouraged their father to take them out without me and ignored any teenage rude comments directed my way. In time they came to trust me and ask me for advice knowing that I always encouraged them to talk to their parents when they had problems and offer to be there if they needed any support or help.
So you see, if you pick a battle now trying to force things, you can find you have started a life long time war of us against them.
Try to see things from their point of view and how they are feeling instead of how you and your new partner expect them to react and behave.