SIL leaked text messages and letters between my husband and his mother on Christmas

My husband cut his mother off when he was 17 and he hasn't seen her in 19 years. He doesn't have a good relationship with his sister because his sister blames him for them getting taken away from their mom. My husband's mom was inappropriate with him from a young age and he didn't understand there was something wrong with what his mom was doing. He thought he loved her and she loved him. When he was 13 his friend found text messages between him and his mom and told his mom and she reported them to CPS because of how sexual they were. So he was taken away. His mom got arrested but was able to get out on probation and they secretly sent letters to each other that got intercepted eventually by his foster mom. He was 14 when he was sending these and after a while in therapy he realized how wrong this was and how he has been abused. It took him a while but he got there. He was upfront about everything when we met. My husband and his sister got into a fight before Christmas and she somehow had him and his mom letters. She also had their old tests but she's still in communication with his mom so that's probably how and she leaked them on Facebook and sent them to my family. My family wasn't being judgemental. My mom reached out to tell me she got this sent to her. Nobody is judging my husband but he's embarrassed himself. He's ashamed of it and refused to go to my family party for new years. They aren't judging him though. They know he was an abused child but he's not wanting to face anyone.