Where?????????????? How do you know she wants help? Just because YOU think she wants help doesn't mean squat. Druggies have their own mindset goin on and until they wanna do something about it then they will continue to live their idiotic lifestyle ( I know all too well )........it's nice that you're concerned and I'm NOT telling you to give up on her, BUT don't be surprised of she "agrees" to help but then two days later id back to the same shenanigans......Good Luck!!
There's homeless shelters, youth hostels (depending on age),churches,local police station should have alternatives. The question is.. Does she want to be helped? Can't help those who won't/can't help themselves :( Good Luck to you & your family.. ......There's probably also some sort of emergency hotline phone number you can call to help with emergency housing.
I'm sorry for your family. Thing is, no one can force someone to get help. If she's doing drugs, she nay not be willing to accept help. I sure wish I knew what to tell you. I will keep your sister and your family in my prayers.