my ex fiancé is seeing my cousin

This is something that's bothering me quite a lot. My ex fiancé (we were together for 5 years, and we've remained friends for the 3 years following) has recently told me he is beginning a relationship with my COUSIN.

Right now I feel very betrayed and hurt. He's his own man, I do not think I have any right to say who he can and can't be with, but I feel very disrespected on her part. He came to me to ask how I feel about it, after saying I was very unhappy and would want nothing to do with either of them if they were to go ahead and get together (which is well within my right) he flipped out, said some very hurtful things about me and then told me he didn't come for approval and he's doing it anyway. Good luck to them, I hope it works out - but it's just wrong and makes things uncomfortable for my whole family as we are all very close and everyone's always seen him as my fiancé/ex/ and then friend. 

Any advice on how to get my head around this as I feel a bit betrayed by them both and it's bothering me quite a lot.

Thank you for any responses.

Anna