Advice

Ladies I need some mom/sister/friend advice. My heart is at war with my head and I don't know what to do. I am at war with my self as whether or not to fight my mom for custody of my little sister. The environment my mother creates is toxic. My little sister stole my moms phone to call me tonight and hid in her bedroom to call me crying and said "please tell me everything is going to be okay, even if its a lie." This absolutely broke my heart. What should I do? I love my mom but it also breaks my heart to see and hear my sister like that. I live 2.5 hours from them and across state lines so i fear the worst... Any and all advice appreciated.