How do I go about getting temporary custody of my little sister who is 7?

I am 21 years old. I was kicked out of my mother's home just before my 17th birthday after a fight with her about money. I was later invited back home and I refused and have been on my own since. I am engaged and have a very stable job working in a youth home and will be attending part time college in January of 2015. Currently my mother and step father have no gas in their home. It is starting to get cold which also means my sister is unable to shower, or have a meal that isn't cooked in the microwave. I just gave them $300.00 to turn their electricity back on. They are very close to losing their home which I have been helping as much as I can to help them avoid. My mother yells and swears constantly at my sister. Also threatens and spanks her. Although, I was spanked as a child, and I do not have a stance on how I feel about it, but I watch my sister literally cower down when she gets yelled at. The spanking is a little more drastic than it should be. She is now trying to get my sister put on medication for ADHD because she simply can't handle her anymore. My mother puts TV on for her and that's where she sits after school. Wouldn't you be a little wound for sound if you sat in a classroom all day and then couldn't even rum and play without getting yelled at when you got home? I have my sister every other weekend. I will admit she is a little more active than other kids, but we run and play and go to the park and do fun educational games together. She cries when she has to go home. I do not wish to take her from them. I would just like temporary custody until they can get on their feet. With the money I give them, I feel as though I am already financially caring for her. My fear I that I will not be granted custody and she will go into foster care. I do not want a messy battle. Just a better life for my baby girl.