If you mean together in a sexual sense, it is highly illegal and could subject you both to criminal charges.
Well, if you know that she has a boyfriend that's great. But never let your parents know. Let your sister tell them. Talk with your sister and convince her to tell your parents.
Your sister needs help. Tell your parents.
Parents never tell my little sister/brother and her mates off when they are annoying me because parents either feel embarrased to shout at your little sister/brother in front of her friend or that they feel guilty telling off your little sister/brother's friend, what you have to do is tell your parents what your little sister and her friend did and hopefully they will tell your little sister/brother off for doing so, that's what happens when my little sister annoys me
Tell your parents and let them handle it.
If you think your sister is at risk or in danger you should tell your parents and not cover for her. If it's something un-harmful you should
Tell you brother to knock it off. Tell your parents.
Tell your parents.
Tell them if the don't tell you, then you won't tell them your school grades. Or tell them you won't do anything they say. OR tell them that if they tell you, then you will babysit you brother or sister and if you don't have a brother or sister then tell them that you won't tell them their surprise for their birthday NEXT year.
tell your parents and show who is boss to her
Dig up some dirt on her and threaten to tell you parents.
Talk to your parents about it. Don't whine, give specific examples and tell them that it really hurts you. They will talk to your sister about it so she knows she is hurting you.
Don't get into the habit of being a tattle tail over everything your sister does. If she is doing drugs; sneaking out late at night or something that may hurt her then you tell your mother what is going on. Your sister will not be happy with you, but it could save her a lot of pain and heartache in the future.