Getting custody of a runaway?

i was in foster care for 5 years i aged out and my younger siblings got placed back home. the case has been closed. my younger sister ran away yesterday. i know where she is at. but the problem is she doesnt want to go back home. My parents are still on drugs and she is afraid what they are going to do to her, they already threatened her multiple times on her cell phone. i want costudy of her. the cops said they cant do much. what can i do?!? i already got a charge from this because two drug addicts said they seen her in my house which they didnt and the cops dont believe me, i told them i have a right to face my accuser so they told me if i wanna play lawyer i can go down to, so i got obstruction of justice. we dont know what to do, how can i get her. i have my own house im stable and drug free. she is 16. i have my own kid. i have an extra room.