Fucking PISSED

So my 17 year old sister decided to make me promise I wouldnt tell grandma she was going to runaway because she got in trouble for talking on her cell phone at 3am. My sister, has always been and always will be my little girl. Im the one who always has protected her, Im the one who when our alcohalice mom wouldn't get from the couch for a month outside of getting another beer, would take care of her. Now I understand if she doesn't want to be with grandma anymore, but her all of a sudden wanting to runaway to brokylin to live with some guy shes never met, is not going to work for me. I would rather break this promise to her and tell then have the chance of my baby, getting raped or murdered, because all that will lead to is me going to jail. Now let me add in, I love her to death but shes a very imature 17 year old, she can't cook or do her own laundry so hgow does she plan on making it? She's not ready. And I really thought she did it tonight because she hasn't answered my texts in two days and always sends me and the baby a goodnight message. So she finally texts me saying "im in hell save me" and then she said she couldn't talk and she was getting her phone shut off in a week and will get a new one. Now when she told me she was running away, she mentioned how she was going to get a new phone so she could not be tracked down.  So after trying all night to get a hold of someone I finally got a hold of grandpaand he said she was at a church thing with grandma which made me feel alot better.  So she is really planning on doing it but hasn't yet and I am calling grandma to talk to her about it as soon as I am done writting this. So anywayas, I told my bf the whole thing and he said I can't control her forever, and I was like yo fuck you, its not about control, but if she goes there and ends up getting raped or murdered, Ill be the one ending up in jail, and I can gurantee he will care then. This whole things just pissing me off.......