I Need Advice

So my younger sister's birthday is coming up. I want to invite my boyfriend as well, but my sister is having a joint party with my cousin who happens to hate my boyfriend. (FYI my sister is 10, I am 18, my boyfriend is 20 and my cousin is 22).

My cousin and boyfriend have a long history. My boyfriend used to be the manager when my cousin, him and I worked together. They got into a whole bunch of nasty arguments, and he wasn't a very nice person then.

I thought my cousin was over it as it has been a year and they have attended parties together. But yesterday my sister said to me that my cousin was going on and on about how much she hates him when I wasn't there, so obviously she isn't. I realize that her birthday party is her special day as well and I don't want to ruin that for her. I didn't realize she was having a joint party with my sister till yesterday. I feel really bad if I don't invite him though because he always invites me to his family events. What should I do?