Hiya love.. sorry for delay in replying was in the bath

I don't know your sister so I am not sure what is really going on with her personally..
however I have done work around domestic Violence.. althought it is very difficult for an 'out sider' (by this I mean someone other than the two people within there relationship) to understand why somebody would stay in an abusive relationship.. she has clearly invested a lot.. and see's more to him than just the 'bully'.
you wouldn't let a stranger walk up to you in the street & hit you.. would you? but perhaps if someone you loved very dearly in your family did it.. your child/a sibling ect did although you may well be VERY angry and upset with them.. it would feel very difficult to just completely cut them out of your life..forever!
Your sister at 19 is still very young.. when this the man come on the scence?
With DV it's not just about the 'hitting' part.. often a lot of work is done by the abuser before this to distroy there partners self esteem.. perhaps your sister feels she can't leave him if she is feeling badly about herself..
& maybe she is lying about if he is the dad or not.. perhaps she is scared of being judged for making foolish mistakes.. no-one knows what really goes on behind closed doors..
it'd be lovely if we could just make someone we love leave a relationship like that, but really something has to click in there head when they get to the point that they relise that it's never going to get any better..
if she has had No contact with the family for over a year, perhaps he does have a lot of control over her? I'd say rather than making any negitive comments about him (even if you really feel it) just let her open up to you.. she'll learn she can be honest with out being scared what you are going to think/say to her.. because i'm sure she must know deep down things are not right..
Also being pregnant/having a young child perhaps she is feeling especially vunrable god only know's what he could be saying/doing i.e using the child like a weapon..'i'll tell social services your a bad mother & they will take the child' or ' you can leave but the child is staying with me, but your welcome to go but dont think once your gone you can come back' ect..
of course I am only sermising.. I don't know either of them and only have very little info to make a guest about..
all I would say really is keep you heart open for her.. let her know you are there no matter what, and when she knows she is safe within your relationship perhaps she might believe there is more to life for than what she is suffering?
I know it's hurtful but i'd try to take her 'lies' with a pinch of salt if she is in an abusive relationship she is prob very messy emotionally and I doubt that it would be a personally refelction upon how she feels about you..
hope things can work out! x