What should I do? I think my brothers girlfriend is cheating on him!?

My brothers girlfriend charged her phone in the room me and my girlfriend were in and she went in the other room. She kept getting texts so we looked at them and she was talking to a bunch of guys and flirting, but we didn't tell my brother since there was no real evidence. Well my brothers in Florida for a week and his girlfriend was over this weekend with my mom and cousin and she must've been logged on skype on my laptop cause shes logged in now and I can see all the covos she had last night at her house. Well I read one and the guy said i was suprised when i walked in the room you got some nice features but the other guy she was skyping naked with. It says hes from texas and we live in pennyslvania. They were talking about showing eachother and he said he needed to see stuff to get hard and she was telin him she was on her period so she cant show that. I don't know what to do cause my brothers in Florida and I don't want him to say something to her if I'm wrong or ruin his tome on vacation and should I tell my mom?