MIL cheating on FIL with an abusive POS

My husband has an extremely complicated relationship with his mom. She's an extremely selfish person who doesn't care about anybody but herself. She left my husband's dad for her now ex husband who beat the fuck out of my husband when he was in high school. He was also doing drugs and drinking around my husband. He got super drunk and high and violently SAd my husband when he was 16. She did leave after that and apologized to my husband's dad and they got back together and remarried after she divorced him. I work as a surgical nurse and once of the patients was my husband's former step dad and his mom was in his recovery room kissing on him. When she saw me she about shit her pants. When I went on my lunch and was going to the Starbucks in the hospital she stopped me and said if I tell my husband or her husband she will have my nursing license for breaking HIPAA. I told her straight up not to worry. I would risk my nursing license for her trash behavior but why not just leave my husband and his dad's lives instead of cheating and putting them through this. She told me to mind my fucking business. We already agreed to go see my in laws for Christmas and I don't know how I can when I know this and legally can't say anything. She's full on threatened my nursing license and said she will make sure to ruin my medical career. Why would I want to spend my Christmas would her but my husband is dead set on seeing family and now I feel like I'm lying to him.