LW1: Jeez, just explain that the reason you call him Jack is because he's not your dad. "We have the same mommy, but mommy was married before to another man, and that man is my daddy" (does she not know that mommy was married before, is that the problem?) Don't tell the girl that you call her father Jack because you're a grown up-- it's not true, and likely caused more confusion on her part. Give your little sister a little credit.