While he will probably not be able to get full custody with the domestic violence, he can make your sisters life a living hell. Have her write him a letter, telling him that the babies father isn't him and she has no intention of getting child support from him. Better yet, have a male freind step up and take the blame. This will let him off the hook for any child support which is probably why he wants custody in the first place. It's all about control in these cases, why should she let him control her for the next 18 years.
Violent parents can do so much damange to a child.
I firmly believe that no child should be subjected to a violent parent because while they might not physically abuse the child the emotional damage can do so much more and is impossible to prove.