it's weird

my bfs cousine turned 17 and she already has a body like a full grown woman and she has a better body than me. And my bf told his friends while I was there that his cousine has a very good body and everyone just looked weird at him and he tried to justify himself by telling me to agree. Then our friend came up to me and told me that my bf for sure likes incest pxrn. And now I can't unsee the fact that my bf thinks his cousine is hot. She also wears a lot of tops showing off her cleavage and I often catch him look at it. That's uncomfortable, but idk how to talk to him abt this topic.