am I crazy or?

Ok, so me and my ex broke up over a year ago. But we have a child together so we're in each other's lives... but lately he's been talking to one of my other sons aunts. Like hanging out with her and probably sleeping with her. This pisses me off so much. Like it's not the fact that he's sleeping with someone else, it's that he's sleeping with my my other sons dads sister. Like wtf. I don't want my sons to grow up and get picked on in school for being brother cousins. He thinks I'm over reacting and being crazy. But I think it's completely disgusting and disrespectful to the kids.