i would definately seek legal advice.
Im a bit confused by your question though .... if he thinks shes had an abortion, then surely theu arent together, or he would see her 5 month bump? If they are not together, why can she not raise the child on her own? Even if you became the legal gardian it would not stop him having rights as the father, so i cant see it would protect the child any more
you say the legal gardianship is because the father is abusive, but i cant really see how you being gardian will help that? unless you live on an unaccessible island or something. Soryy not to be of any help, but i would think you would have to at least apply through the courts, if only to give the father a chance to contest, and to protect yourselves if your sister changes her mind after , say 5 months