Boyfriends step brother is my ex who cheated

I feel like everyone is going to call me a baby or overdramatic. My boyfriend is started introducing me to his family. I love my boyfriend. He's my world. When I met his family I also met his step brother and they are like best friends. Turns out his step brother is my cheating ex and the reason I don't talk to my cousin. My freshman year of highschool I had my first boyfriend. He was my first love and I couldn't have been more in love with. Well he gave me my first heart break. He cheated on me with my cousin and I don't talk to my cousin anymore. I hate my cousin and have wanted nothing more than for her to have a miserable life. He ended up marrying my cousin and having 4 kids with her. Fuck them both. My cousin has tried to make things right with me but I've told her to fuck off. Every once and a while I'll talk them on Facebook because they're going on 18 years married. I have always hoped their relationship would fall apart. I told my boyfriend who he was and my boyfriend found the whole thing funny until he realized how hurt I was. He asked was I over him and I said yeah. He said good. Then it doesn't matter. But it does to me. I kind of want him to not talk to him. Plus he's a step brother that came around his junior year of highschool. They didn't grow up together. It shouldn't be hard to cut contact. I know I sound like a psycho bitch but he says he loves me and wants to marry me and have kids with me. Shouldn't he be open to cutting off people that hurt me?