My lil sister boyfriend is a real dork, but he is my friend, what to do???!!!

OK, the thing is that I have a lil sister (well...not so little, she's 16)
and she has a boyfriend, normal thing for a girl of her age, im also 16, and we are twins, not identical twins, but we were born the same day, im around 2 minutes older than her.
I have defended her and stood for her in countless fights/problems/anything since I remember and we are very close to each other.
her current boyfriend is a total dork, I have already spotted him cheating and TRYING to cheat on her with around 15 different girls.
problem? the guy is like my childhood best friend.
and what hurt me the most is that sometimes, she tells me "do you think Dwaine cheats on me???"
and with all the pain in the world, I must tell her "definitly not" cuz I actually care for the guy, we are like best pals, garden...
even the other day he said something like "omg I really LOVE that girl" and I told him "dude, should I remind you that you have a girlfriend, and that she is my sister?"
and he tells me "I know man...I love your sister, but I love this girl even more..."
I already spotted him making out with like 15 other girls, I tell him "dude you're gonna have to change or something's bad gonna happen!"
but he doesn't want to listen, im sick of it, im already getting to think that the guy is just with my sister because he considers her a "backup" so if other girl rejects him, he knows he have my sister there "in the storage"
but im damm sure that if other girl tells him she wants to date him, he would instantly dump my sis!

so what should I do? what's more important? friends or family? really need a tip here...
Apesto en el ingles como tu apestas en español
Last edited by GIRbot99 at Sep 16, 2009,
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Do a barrel roll!

barrel roll should do the job....

no, seriously, need a lil REAL advice ¬¬
Apesto en el ingles como tu apestas en español
Relationship thread would be appropriate.

Always protect your family first. You should really tell your sister the truth, as hard as that might be.