who needs sibling issues?

my bf's mother texted me yesterday and told me that her daughter...my bfs (half) sister...is going to be spending the night friday. that took me by surprise. he really does suck and letting me know about plans. well anyway, his sister just got out of rehab a week ago and i guess his mom is already sick of dealing with her. i get the stress of having kids...i just have one, but i mean everyone knows that it is up to no one else, but the mother to raise the child. i am sick of taking care of this troubled teen. she is 16 yrs old and has to be babysat and watched more than my 2yr old. not to mention the huge issue of this girls past. she has stolen pills and money from her mother. not to mention snuck out in the middle of the nite with who knows who and doing who knows what numerous times. who is to keep her from doing those things in my home? with my little girl under my roof? its just not right. so i confront my bf about it. saying that i am not ready to trust her after just a month of rehab...blah blah blah. he gets so mad. saying how im shit talking his family and everything. it is hard to be in a relationship with someone who wont be on my side for anything. well i txt his mom and tell her that friday nite doesnt work out for us. i dont tell her the real reason for any of it. this lady is the kind of person u do not want to deal with. not to mention she is a GOSSIP QUEEN!!! well anyway, she texts me back and says...and i quote...i really feel that u hate when (bfs name) spends time with his family. you two should be proud that (sisters name) looks up to you so much. i just wanted to say "look lady. it is not my job to f-ing sit your f-ed up child ok?" idk. dont get me wrong. i dont hate her and i dont hate it when my bf spends time with his family! i really dont. his sister has spent numerous nights with us...in a cramped apartment. its just hard. idk who is right on this one. help?