I can't make a decision. My 14 year old sister ran away...

My parents and younger sister live 460 miles away from me. My mom calls me this morning and says my sister ran away. My sister calls me from a friends phone and told me that she had a boy over, my parents caught her, and flipped. They hit her and broke her phone. She ran away to a friends house. I told her to go home and I was coming down there. I left work early. Came home and woke up my boyfriend, cause I didn't want to go alone. He's on night shift and flipped on me for waking him up. I begged him to come with me and he said no, that he didn't care and he hated being around my family. I'm scared to go be with my family because I'm afraid he's gonna cheat on me. He broke up with me for someone else in January, then that didn't work out so we got back together in March. I caught him on a dating/hookup site the other day. So...if I leave, I feel like he'll cheat. If I don't leave, I'll regret not being there for my sister. My mom is actually kind of crazy, and we weren't really talking until today. I don't know what to do and I'm so torn. Not to mention, if I go be with my family, I'll have to miss work. Ugh. I want to talk to and work things out with my boyfriend. But I don't want to do that at the expense of my family. I also don't want to lose my boyfriend...I've put so much time and effort into this relationship. I don't want to throw it all away. 
Also, I'm looking for constructive advice, not judgement. Thanks in advance.