Incest inlaws

There were jokes made about my sister in-law having a crush on her 1st cousin. When I told my fiance how digusting it was,he got defensive & said his aunt was spreading that lie. I saw her putting a hicky on her cousin a couple days later & after our last conversation, plus the fact that he also saw it & said nothing, I decided to keep my opinion to myself. But now he wants the 4 of us to get a place together. He hasn't seen his cousin in 6 years & wants him to stay, and acted like he wants this bad. But their relationship makes me sick. I even had anxiety attack when he asked. How can I talk to him about it, without ruining our relationship?

Tuff one there. Some people take it lightly. Try and find out what he really thinks about it and why he thinks that way. His answer will make you able to decide for yourself what you need to do.