My brother is a pervert...

Alright y'all. This isn't about me but its disturbing af and I feel like I need to speak up.

I have a pair of twin brothers, turning 21 soon. One has been dating the same girl on and off for several years. She doesn't have the best reputation but she's a sweet girl and loves my brother. She tries to avoid confrontation and hurting people's feeling to a fault. So, story time....

Last night twin #1 and his girl(we will call her Sam) came to visit. They have been having relationships issues and I wanted to help. They informed me these issues are due to twin #2. Hes been making moves on Sam. Sending her dirty messages, asking for sex, even grabbing her butt. Sam went along when it was just texts to avoid upsetting twin #2. Twin #1 knew the whole time. Apparently things got to the point that twin #2, the horndog, bought condoms to use with Sam. Twin #1 got so angry her put his head through a wall! Now Sam is afraid to explain to my family why my brother hurt himself and they assume it's her fault. Y'all, as the big sister of these boys, I feel like I need to intervene. For the sake of Sam and for the sake of twin #2. I'm afraid he will pressure other girls. I dont know what to do! I want to talk to my dad about it, I just dont know how to approach such a subject.