Aw, big HUGS, Sarah,
You're in a tough spot, especially with those younger siblings to worry about.
Have you talked to your mom about her drinking? If she's like most problem drinkers, she won't want to talk about it, and will say she doesn't have a problem.
YOU, though (and your brother and sister) have a problem. I suggest you try to find an Al-Anon meeting for yourself. Your brother would probably benefit from Alateen. I found Al-Anon to be a real lifesaver--it will help you understand what's going on with your mom and help you get a clearer head so you can decide how best to take care of yourself and help your brother and sister.