I can't watch her do this to herself

I have this friend…were so close I consider her my sister..she met my older brother because we hung out..and now theyre dating. shes 16 and hes 19, which makes it really uncomfortable for me..and also illegal.she was raped when she was 13..and it ruined her life and she has a lot of problems because of it. asnd I found out that she is having sex with my brother, shes smoking pot with him, shes drinking and smoking with him. and I knew she drank when I met her, but she didnt do any of that other stuff. imm afradi to say somethign to her because she doesnt think shes doing anything wrong and im afraid shell get mad at me and go tell my brother what I said to her..and he knows stuff about me that could get me scrwed over if I piss him off, so im afraid to say anything. but im so worried about her especially because of her situation what shes doing isnt making anything better and shes going down a bad road and just making the hole deeper for her to dig herself out of..and a lot of it is because of him but she doesnt think so because they are “in love”(note the quotations) and I dont know what to do..I cant watch her do this to herself..please help me