Do. Not. Get. Involved. Yet.
That's my piece of advice. Wait it out until the custody agreement, then wait a couple of weekends (or weeks, whatever he gets) and get to know that kid. Chances are, if you overwhelm her now with big sister stuff, her mother is going to catch wind of that, and make your life miserable. That wrench does not need to know and start implementing in her young daughter's brain that not only does Daddy not love her, but he's also replacing her soon with a brand new baby. People are twisted like that.
Trust me, I was that kid. My mother was always neutral, keeping her opinions to herself, but my grandparents apparently never heard of that concept.