How would this affect our relationship?

Yes, I feel like I’m keeping something behind her back. She always wants me to be more open, even though I’m pretty open and honest with her.

I don’t know if it’s worth telling, it will just cause unneeded drama. If the topic comes up then tell her. But to just tell her “oh, by the way, my friend showed me those naked pics you took…” would be unnecessary. That’s just my opinion. I don’t know what good could

Could come out of telling her

Yeah, I wouldn’t just randomly say it. Lately she’s been trying to become friends with him again (just friends) and I think I would just say it, next time she brings it up. Saying something like “Well he doesn’t care about you because he showed me and a lot of people your nudes, and wanted a threesome with your sister.”

Okay now you’re just sabotaging a possible friendship… For what reason? Are you jealous of him?

Well I doubt “friendship” includes showing your friends nudes of you, and talking about how much you want to have sex with their sister. I just don’t want her losing interest in me, so yeah, I guess :/

I wouldn’t do that if I were you. Making decisions for her isn’t something you should be doing and it’s going to accomplish the exact opposite of what you wanted.