What to do if your exboyfriend is dating your cousin?

I'm pretty sure it's illegal to date a relative.

I think it is but if u really love each other and didn't know that u were related then i suppose its okish but its kinda creepy if its your brother or sister.

Many people in the early years, like for example Edvard Grieg, married his sister. It wasn't illegal it was just frowned upon.