It is always a strain on a family when one member is making poor decisions. I am so sorry your family is dealing with such hardship! My suggestion is, as much of a burden as it would be, try and find out if another member of the family (who lives in a totally different area) would be willing to take the girl in. It is very difficult to get away from drug habits when there is easy access. Also, as much as your sister wants to help, as long as your niece sees her as the enabler and not an authority figure, she will be unable to make a real difference.
I actually have custody of my little brother - he's 16 - he had terrible depression, was overweight, and had had a few suicide attempts. Stayed back a year of school, also. His father has a drug problem and, after finally convincing my mother to let me try and help, they signed residential custody over to me. After a rocky start for the first few months, we have hit our stride. It has been about a year total, and my brother has lost over 50 pounds, gets himself up and on the bus EVERY DAY, and has straight A's this marking period. I set myself up as an authority figure and did not allow myself to waver, and although it was scary, it paid off. Love does conquer all - my mom's love for letting him go, and our love for taking him in.
Good luck!
I actually have custody of my little brother - he's 16 - he had terrible depression, was overweight, and had had a few suicide attempts. Stayed back a year of school, also. His father has a drug problem and, after finally convincing my mother to let me try and help, they signed residential custody over to me. After a rocky start for the first few months, we have hit our stride. It has been about a year total, and my brother has lost over 50 pounds, gets himself up and on the bus EVERY DAY, and has straight A's this marking period. I set myself up as an authority figure and did not allow myself to waver, and although it was scary, it paid off. Love does conquer all - my mom's love for letting him go, and our love for taking him in.
Good luck!