14 year old sister's ually active with 17 year old thug and gang member?

I think you should tell your parents. She is only 14 and as he is 17, if they had , he could go to prison. She probably thinks they are love's young dream but that is only because she is vulnerable and naive. Telling your parents may seem like you are gring her up but it is the only way she will get out of trouble. She may hate you for a while after wards, but later on in life she will thank you for not letting her get hurt and for being there for her.