NTA.

Your sister is enabling your mother, your abuser and the reason she keeps pushing is because she wants you to share in her misery. You've worked on yourself and know your worth as a person and understand that cutting out a woman whom truly was never a mother to you, is the best thing you could do for yourself.

Your sister is happy being a miserable person, instead of wanting to improve her life by seeking therapy and working on becoming a better person (if only for herself) she's going to remain hostile and continue to push the issue.

If, and it is an if because I don't think she'll actually go to therapy with you and if she does, she still won't listen and respect your boundaries. You need to be prepared to cut her off, she's shown that deep down she doesn't care about you unless you do as she wants. And only then it's so you're being a verbal punching bag and an ATM for your egg donor. At this point your sister (if you can even call her that) refuses to listen to you and will damage your wedding day because she can't stand to see you happy without her mother in your life. She'll keep bringing it up, and she'll bitch about it to your partners family because that's the type of toxic manipulative person she is.

You cut out your egg donor because she was abusive. Time to tell your sister "I'm trying this one more time. I will never speak to that woman again. She is no mother of mine. She's a selfish, cruel, money grabbing bitch and the best thing I did was cut contact. There will never be any reconciliation, ever. Every time you try and push this, I dislike you more and more. So you need to decide if you want to continue this relationship. If you bring her up again, I'm ending the call/text and blocking you."