You don't say how old you are. This could be a determining factor. However, rather than thinking of custody, could the problem be solved by your dad granting you temporary guardianship over her? This would allow her to live with you and for you to make decisions for her. It is not a drastic step of changing custody and may be more palatable for your father. Otherwise, you do not have standing to pursue custody of your sister.
Answered on Sep 03rd, 2012 at 12:10 PM