There are obviously a lot of factual details not noted in this question. If you (or anyone else) wants to consider stepping in for your sister, you should consider all available possibilities -- temporary or permanent guardianship, an actual custody order, or even a termination of parental rights followed by an adoption. You can get an explanation and background on all of those things on our website:
http://willicklawgroup.com/guardianship/
http://willicklawgroup.com/child-custody-and-visitation/
http://willicklawgroup.com/termination-of-parental-rights/
For a more complete analysis, of course, you should make an appointment with a family law specialist, give a complete factual history, and go over all possibilities.
Answered on Sep 07th, 2015 at 10:26 AM