L.M.
For better or worse, this is your neice/nephew's FATHER. Whaterver else he may or may not be, this man will be important to this baby, either because he is there or he ISN'T there. Are you actually saying you don't want the father of the baby to be at its birth??
Its time for him to step up, big time. As the Aunt, I wouldn't want to stand in the way of this man stepping up to be the father the baby AND your sister needs. Unless he has physically hurt her in the past, you need to give him the room to either assume his responsibility or fail.
You are a grown woman, who I'm sure can handle the situation. Be the bigger person, and pray that the birth of his child helps him grow up and take responsibility. (When he fails, you can kick his keester then)
Whether you like it or not, this man is part of your life now via the baby. Don't create a wedge between him and your sister/the baby. Its not right.