I'm the Aunt, can the father take baby?

My sister recently gave me her baby, as she no longer wants to be a mother (yea, she's wonderful person *sarcasism*). The baby's dad IS NOT on birth certificate, does he have any rights to her? Can he try to take her away form me if I get custody? I don't know much about him other than he's a woman beater (beat my sister when she was pregnant), and has another child that he is not allowed to be around (per the courts). Can the dad do anything to screw up adoption or custody of my niece? I am going to start with guardianship, but that's all I know where to start.