5 Answers

I would tell your sister that you'd go with her to see your mother.  Tell her you want her to be the one to tell your mother the truth.  Either she does that or you'd be the one to tell your mother.  Give your sister the choice.  If she asks why, tell her that you had already agreed to tell mom if something bad happened.

If nothing went wrong while your sister's friend was there, keep it to yourself then blackmail her with it. ;) Unless the friend was a boy and they were in her room with the door closed, then tell, tell, tell!!

Go ahead,snitch on her.Who cares if she hates you for the rest of your life.I mean she's only your sister.It's not like she was a friend or anything.

If you were left in charge, then you should tell your mother. If you were not in charge, you should tell your sister that you will tell if it happens again.

charge her money to keep quiet