So great that you are reaching out for help and support. My first suggestion would be for you and your sister try to find an alateen meeting..they can really help.Also since you guys are under age..is social services involved at all? Do you have a grandparent,father,teacher,reliable adult in your life you can go to?
It sounds really hard and I can hear how much you love your mom, but you are still legally a child and could probably get some help from some outside resources so you and your sister can focus on school.