I know my sister-in-law was not pleased with her sister getting pregnant at first. But by the time she was due she decided she just needed to be there for her sister and support her in any way she could.
I think that's what you need to do for your sister. Sure the father may eventually become not much of a father. But at least she will always have you, her sister. Also, since the father is not very supportive of this pregnancy she definitely needs SOMEONE on her side.

As far as what to watch for in the "father", I have no experience with that. So I can't help you there. I just know you should be there for your sister. She may need you a lot through this pregnancy. And trust me, once that baby is born you are going to love him/her so much you can't stand it!

That's my two cents anyway. And congrats AUNTIE!!