Advice please..

My 15 y/o sister is out on the streets. My mom put her out because her boyfriend doesn’t want her there and told my mom he’d leave if she came back. She blames it on the fact that she can’t afford bills without him. She’s stayed with my aunt and she’s kicked her out and she’s stayed with my grandma and was kicked out of there too and isn’t allowed back to either place. She is very ungrateful and disrespectful.. I feel like most of it comes from wanting my mother’s attention. She wants to smoke weed & run off with boys all the time. She lived with me for a little while but is super disrespectful, didn’t listen, didn’t do anything to help around the house and snatched up my 4 month old and could have hurt him honestly. She’s kicked out of the school district by my house so she’s have to go to school at another school district. don’t want her to get put into states custody but I really don’t know what else to do at this point