My(F24) dad (M50) was cheating on my mom (F43) and I can’t tell her because of how I found out.. need advice?

So here’s some background on everything, almost 3 years ago my parents were having issues where she suspected he was cheating or about to cheat. He was close with this girl at work, and later down the line Someone told her that he got an STD test because that was this one girls rule before having sex. Come to find out, he did get an STD test without telling my mom. It all added up and he admitted to he was going to do something. They argued for a while and one night they went to a bar and argued during the drive, my dad was drunk driving and they crashed. My mom lost fingers and completely shattered her elbow bone. She is now disabled and unable to work. She doesn’t make enough on disability to survive by herself. They have 2 young children to care for still.

Fast forward to today, for months my mom has suspected something was going on. She’s been ranting to me and some of it seemed fishy, but a lot of it seemed crazy. But, a woman has intuition about these things. She would always ask him about the sketchy things and wanted him to call her on his breaks to gain the trust back. (He doesn’t do anything on his breaks anyways) a month or so ago he started rebelling and getting annoyed with her and saying he needs his space and he wants his breaks back. She thought it was weird because he doesn’t do anything but sit in the car anyways. Well, two weeks ago he came home and told my mom that he wanted to separate, but still wanted to stay there and help with the kids. He said he was tired of constantly being accused when he’s not doing anything and that she needed therapy. She was devastated and begged him if she went to therapy they could make it work. She can’t afford to live on her disability payments and take care of the two small children. He said he wanted to make it work, but took his wedding rings off the next day. She has been asking him to put them back on if they’re making it work. He just kept saying he wasn’t ready so she told him she needed to know what was going on and if he didn’t put his rings back on, then he needs to leave because she’s not sitting around waiting and being his house wife.

I have been in the middle on how to feel until today. My brother(19) works with my dad, and my dad is his ride to work since the car is in his name (even though it was gifted to my mom smh) My brother told me that on the way to work, my dad said “just between you and me.. me and blank are seeing eachother. Like dating” (SURPRISE, it’s another girl he works with) My brother said I can’t say anything to mom because dad is his ride to work and he doesn’t want him to stop taking him. I’m so angry and want to tell my mom because she thinks she’s crazy and she ruined her 26 year marriage.. but she didn’t. She’s been crying to me telling me how crazy she feels and that she should just go check into a hospital. And I’m sorry, but she’s been right this whole time. You don’t just start “seeing someone” only two weeks after you decide to tell your wife you want to separate. He has BEEN seeing her. I am angry asf. How could he do this to her after he caused the car accident that ruined her life and now she can’t even work!!! I’m disgusted and I don’t know what to do with my feelings. I want to tell my mom SO BAD but I don’t want my brother to have to face the consequences. Idk what to do. I want to text my dad so bad and just lay into him and call him a nasty and tell him how I really feel.

Please give me advice on how to handle this. I want my mom to know she’s not crazy, I want my dad to know how disgusting of a human being he is. But I only found out because my brother trusted to tell me. He wants to tell her too, but keeps saying dad is his only way to work.