Don't know what to do about falling apart family?

[GuitarTherapy]

i am a college student who moved away from my mom and 2 younger sisters a couple of years ago and i am not sure what to do about my family. my dad left our family when i was 9 and my sisters were 4 and 2 and they havent seen him since. my sisters have no memory of him at all and i think a lack of a father figure may be the root of some of the problems. my mom is a very negative person and she is constantly screaming at the girls and i can not remember the last time i had a conversation with her that was not filled with negativity. we have recently found out that my youngest sis has been talking to this guy who is in her grade but he is much older due to being held back several times. they have been sending disgusting sexual messages to each other over the phone and myspace. also the same sister has lost about 20 pounds in the last couple of months. (school thinks it might be drugs). she has changed and is not the same sweet girl she used to be. what can i do to help my family?