How do I go about this situation?

I’m trying to word this best as I can. So a little back story. From 14-18 years old I was in the custody of my uncle (dad’s brother) and his girlfriend (aunt). My “aunt” is extremely narcissistic and abusive in many ways. My uncle goes to work, comes home, hides in the bedroom and doesn’t interact with anyone unless my aunt needs reassurance that she’s right. 😒

My 2 siblings also lived there too. I left when I turned 18 because of how we were treated. My sister came to live with me when she turned 18 and my brother still lives there (he is almost 20). My sister and I want nothing to do with them. This has made the relationship difficult with our brother because it’s always my aunts way or no way. We’ve told our brother that we don’t want to be around. I don’t think he has told them we won’t be around them because we get asked to come over frequently. I always make up some excuse but I’m tired of it.

Today my brother messages asking if we would like to come over for Mother’s Day. I know that my aunt told him to ask. It angers me that she tries so hard to try to bring us back into her life (my uncle literally doesn’t care about anything so this is all from a woman who came into his life and so happened to take us in because money was involved.). If I want to talk to my brother or see him she tries to take it over and make it about her. I don’t want to see her for Mother’s Day. She’s never been a “mother”. There’s never been a change in her behavior. Even when I thought things were different she would turn around and do something that made me realize why I wanted to be far away in the first place.

How do I say I don’t want to ever attend anything they invite us to? My fear is that she will see my message get angry and somehow force my brother not to talk to us. It hurts, all I want is my brother away from there. I don’t think he realizes how wrong things are because of how we grew up. It’s all he knows. I wish that I could erase my aunt and uncle. My sister and I both just want our brother in our lives. I’m trying so hard to be respectful about things but I feel like no matter what I say she won’t like it. This scenario happens often and I thought it would be helpful to share with people that understand so I can set better boundaries. Thanks in advance.