It's hard to take away full custody from a biological parent absent abuse.... your questions suggests abuse (verbal) and maybe physical abuse.  It may or may not be enough to get his rights terminated, but it should be enough for you to file a suit to intervene.  If the parents are divorced, you would need to file a suit in that lawsuit.  If they were never married and there are no custody papers, then you would have to file a new suit.  Texas actually has a provision in the family which allows siblings formal access to minor siblings.  That would be a good starting point.  If you have an aunt who is will to fund it, then find a good family law attorney to work with.  If the abuse if really bad, contact CPS immediately and get her out of the situation sooner than later-- also let them know that you or your aunt are will to provide her housing while the investigation is pending.