I need answers!! Its worse everyday and idk what to do?

Okay, My family is not the best. not close and nothing but drama. i know every family has its own faults but this is out of hand, my sister and mom has never had a close relationship and my mom has tried so hard to change that but my sister continues to be a b**** to her. Recently something happened and mom said call me and she just says no, don't want to talk! That's pretty s***y that she would say something like that to her own mom. she has always been this way. I just want them to have a close relationship. I hate to see my mom hurt and she easily gets her feelings hurt but my sister doesn't seem to care. I just don't know what some things that we can do to change this! It is a mess. I have never seen a daughter and mother relationship so bad.