Abusive parent's birthday coming up

I'm sorry. My stepmom abused me and she's still married to my dad and they have a daughter together so she's not going anywhere and will always be in my life. It's hard. Really, really hard. I still go out with them on family things- Easter, Christmas, their birthdays etc and it can be hard to get through. I know as an adult I have the choice not to go, but, it's not that simple- I want my dad in my life, even though I don't want my stepmom in my life. And I want to be in my little sisters life too. So. I kinda have to hang out with my stepmom too because I can't really just be with my dad and sister and not be with her, you know? It just sucks. I'd avoid him if you can, but if you can't... maybe you have a way of making it clear that you're not the same kid you were when you were being abused, you're older now and he has no power over you anymore. If he ever says or does anything wrong, now you can loudly and clearly tell him that's inappropriate and you aren't going to allow it. It's scary though. I have trouble asserting myself like that sometimes. I don't know.