Grew up in blended family my lovely sister (half for the record) hated me when she was a teen. But now we are both adults and she apologised for the way she treated me when she was younger and are very v close
She said she was angry and believed all the nonsense her mum spouted and directed her anger at the wrong people.
She is also now NC with her mum for years now. And she had a shit ton of therapy- where she was told quite categorically that although she was a child of a divorced home, that she was now a adult and just because your hurt doesn't mean you go around hurting others and it be justified
I can see a lot of people excusing this type of behaviour as if the people we are talking about aren't adults and are punishing their blood for existing and that's all fine and dandy 😵💫
People can and do make other choices that isn't well of course you don't want to see your half siblings as a adult you must be so jealous of the new family. I would find that really bizarre if someone said that to me and I know my sister would feel the same.
That said I never pushed my sister in regards to a relationship and let her make the first move. I always felt v sorry for her - especially when she had to pretend to hate me when mum collected her and the others.
My mum framed it to me at a young age that me doing that and not letting her see my hurt as a kindness and protecting her because I also knew her mother would punish her if she had a inkling she even acknowledged me.
Op I totally get where your kids are coming from, my vote is to acknowledge your children's pain as valid (because it is) and that equal to the older siblings pain and I would say to them dad can't cut them off because your all his kids. And I would leave the older steps relationship to dad and if asked I would tell them it's hard to have a relationship with you when your willing hurt my children over a matter that they didn't cause.
Maybe they will change maybe they won't but my god they you bet if they need a kidney in the future they might have the door slammed in their face. Choices have consequences and all that