what do i do, when my mother is an alcholic/druggie?

ok heres the deal, im the oldest of three girls, my mother is a alcoholic when shes off drugs. i basically raised my two younger sisters. we all have different dads, and only one of them has had any part in our life. my youngest sister has lived with me off and on for the past 6 years, till i joined the army. since i left for basic she has started failing in school, picked up smoking weed, and cigarettes, drinking alcohol, and now having sex, shes 13! worst of all her mother was allowing her to do it!!! i heard through the grape vine my mother is starting to get some help and doing better but my sister is already going down the wrong path. our mother is supposed to be going to rehab for 30 days, leaving my sister with some guys she just met (like within the last month) from church!! im leaving for iraq soon, and i dont know how to help my sister. what can i do? i have tried to talk to her, and bribe her. shes at the age where shes starting to rebel, and i dont want her to turn out like my mother.