Found out something about my brother that I just can't believe.

So a bit of backstory. My brother is 28, I am 31 and on holidays my mom tends to drink a bit too much and gets hateful and upset towards everyone. My brother and mom live with my grandma due to financial problems. My brother has always been closer with my mom and just hangs out more, they both love music so they bond over it. My brother bears the brunt of the arguments on holidays because he lives there and I get to leave and go home. Well on Thanksgiving it was no different. She drank some but the arguing didnt start intil I left and got home. It got really bad and then my brother sent me a voice messages crying that he may get kicked out. A bit out about my brother, he has been arrested a few times and made dumb decisions but hes genuinely doing good rn outside of trying to find a job. Well after the argument, i couldn't get ahold of either of them on FB messanger or by phone. Eventually I got ahold of my mom and I was under the assumption she had started it. She kept telling me my brother had a problem she didn't know how to fix. He does have anger problems but nothing that causes him to get violent and I asked if it was ANYTHING illegal he had done and she kept saying no. She said it would be something that would make me hate him. Eventually I stopped pushing trying to find out but she told me anyways. She said for the last 4-5 months he has been texting her trying to do things with her. Sexual things. She said she could send screenshot of what he sent. She said he stopped for awhile so she thought he got over it but didnt. He is an amazing brother and its so close to Christmas and all our friends and family meet at my grandmas. I know if this came out it could ruin Christmas. I havent told my gf because I know it would horiffy her. I havent texted my mom anymore and ive barely messages my brother. Only when he messaged me did I send anything back. He doesnt know I know. He isnt right in the head thinking that way and I have no idea what to do. I feel like it'll ruin my relationship with him and mess the holidays up. Any advice is welcome. Sorry for the rant.