Should my MIL know?

I fe awful not going to Holidays with our daughter. I absolutely love my MIL. She's an amazing person, but it's who she's invited which is the reason I'm not going. My husband was molested by his cousins when he was younger. His mom doesn't know and how I even found out is a longer story. It started with a post I made on here about my husband joking and teasing me for pooping during labor. My husband isn't a bad husband but he doesn't know when to stop when it comes to his jokes. He kept it going and in front of his family and his mom showed me his old bed wetting chart from when he was 15 and told him that I had a baby and what was his excuse. He went the bed until 16. A while after that whole situation (and he did stop making fun of me pooping ruring birth) he did mention getting sexually abused by his cousins and they is what we linked to him having accidents for so long. Thing is, he never told his mom. So his mom just had these cousins over and I don't want my child around them. My husband is fine with not having our daughter around then but since he hasn't told his mom about that I feel like we look like jackasses when we don't go. It hurts my MIL feelings and she has no idea about everything and why we aren't coming to Christmas, but my husband isn't wanting to talk about it because he knows his family won't survive that and the last thing his mom needs is that drama. I think he hasn't come to terms with everything and that's why he won't tell his mom but I think she should know. She probably things we just don't want to see her for Holidays.