How can we help my SIL

My husband and I are desperately trying to help my sister in law (his little sister) get out of doing drugs but we don’t know how to help her. She’s 16 almost 17 years old and her mom doesn’t really care what she does. Today my husband got a call from a lady who turned out to be the mom of the boy his sister is currently dating, they’ve been going out for about a year now, on and off but the mom was saying she doesn’t want my SIL around her son, she knows they do drugs together and just wants them apart, she said she’s asked my SIL multiple times to leave her house and she does not and she doesn’t want to call the cops on her (this lady is too nice), another reason the lady called my husband is because my mother in law does nothing to control my SIL, she pays no attention to her at all. What can we do to help since we’re on a short leash considering she doesn’t live with us nor do we have full authority over her. This has gotten too far but before it gets bigger we need advice