To be brutally honest that warrants a punch in the face even if he is your friend. I mean your sister is frikken 12! LOL when I was 15 I didnt look at 12 year olds like that!
tell her to quit mac'n with your friends...and tell your friends to kiss someone else's sister
they are kids let them enjoy but just kissing if it turned into some thing else you need to step in
kiss his sister then his mom.
dodgy as - depending on circumstances and how he dealt with my raising the matter, i might end the friendship - that kind of sexually aggressive behaviour would get you killed in a great many places
but you should talk to you sis first