Since you asked what you should do, I'll give you my opinion.  It's no better than anyone elses, but it's one thought.
Dump your bf.  He wasn't drunk (not that that would be an excuse). "'she offered it ona plate, would you say no? "  YES.  She is responsible for being trampish and he's responsible for being untrustworthy.  He has no proper place in your life.

Be careful of taking comfort from your sister's bf.  He's 17 and she's a 13 year old trollup.  Why do you think he's in the relationship?

Unfortunatley, your sister will remain your sister and somehow, sometime you should try to mend the relationship with her, but I don't imagine you'll be up to that any time soon.

I'm not big on "telling" parents on siblings, but your sister needs some oversight.  A 13 year old should not be dating a 17 year old and finding her on top of your 18 year old bf with her bf down stairs, tells me this girl is headed for trouble.  Perhaps at this point you don't have her best interests at heart.  That's understandable.

I really wish I could say or do something to relieve your current pain.  Please feel free to pm me anytime.  good luck

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leave him alone just get him out of your life if i was you i wud date ur sisters now ex bf he seemed nice

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i agree

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"Brave is knowing that tomorrow isn't a bright and happy future."

First....tell your parents about your sister.

Next.....never talk to your bf again.

If you can't trust your own sister who can you trust?

If you can't trust you boyfriend you never will trust him again.....get rid of him.  He shouldn't be hard to replace.

You do know that what he did with your sister he could be arrested and go to jail?