Can I gain full custody of my niece?

So my sister has just informed me that my youngest niece has been taken away from her. This is her 3rd child that has been taken away and is the 9th child in our family that is going through the system. I told her that I want to gain full custody because I can't see another child in our family being taken away. She said that she has already been adopted, but it was an illegal adoption because she never signed her rights away. Is there any way I can still gain full custody?