Would i be able to get custody of my niece?

well the first thing to do is call your local child protective services. Then you petition your courts for temporary custody (so she doesnt go to foster care) hire a lawyer to start proceedings. it may be that the courts want to put your sister through rehab services. if they work they will give her daughter back to her. If she continues to do drugs then you have another chance to gain custody. This whole process could take YEARS unless your sister voluntarily gives you custody. By the time this all goes through the court your niece may very well be old enough to leave and stay with you on her own legally.