Wondering something

Amom had asked me if I was still in contact with bdad last year. I had 2 other kids with him after I gave up my daughter. I do know she doesn't want my daughter to meet (if they ever tell her) I wondering if this could be the reason why they pulled back from telling her because the eventual question would be about who her dad is. Amom knows he was abusive to me and treated me badly and has put my kids through hell and back and I understand why she would want to keep her from that in the future. I personally am nervous about her meeting him because he has nothing but bad things to say about me. In the future when they tell her and she has questions about her father, what do I say? I would never bash him to her but he would love to relish in it. He never wanted her until after the adoption was final, we were not together. He throws in my face that HE never signed the papers but he never showed up in court to contest the adoption when he was served. In his mind I threw her away and he has never forgiven me for it. I know it would not be right to keep him from her but How do I shield her from his manipulation.This is particularly difficult being since my adopted daughter (16) and my other daughter 14 are full sisters. I thought about talking to him (at least attempting to) about not trying to turn her against me if reuniting comes into play. Anyone had to deal with a bdad who holds you giving your child up against you and threatens to tell them bad things about you if given the opportunity?