Hmm my sister is among the dumbest people I know, so my first reaction would be laughing probably and like me her ass already has smal things oribiting it, which considering my friends (for what ever reason) all look fairly good, would make it even funnier.
In all seriousness though, I have two friends who are a bit loose when it comes to faithfullness and I'd defineatly tell her about that and advice her against dating those two and expecting a great relationship out of it. Appart from that what do I care, I never understood the fucking creeps that think they can decide who gets to date their sister...
I had a fellow student once (only had one lecture a week with that creep luckily) who keep talking to me how he'd protect his sisister and that jazz (I've never seen her and he just started talking on his own o_O)... I never missed my train ever again after that even if I had to leave the lecture 15 minutes early (those weirdos for some reason always feel they have to tell me their live's story).