Oh, it's TOTALLY normal to feel this way, from all sides of the coin. I know my husband has a really hard time when the kids see their bio dad. My husband and I have full custody and do everything for the kids and they don't see it at all, but it only takes ONE short visit with bio dad for him to be a hero in their eyes.
It really rang home how much it bothers him the other day, when our daughter told him "I love you and dad the same amount." I saw this as a GOOD thing, but he was really hurt by it because he is their father 100% of the time, whereas bio dad actually abandoned them, and now only pops in and out when it's convenient to be a parent.
It's totally normal to get these stabs of jealousy. Parent or step-parent, the child is still your child, and it's hard to "share" that priveledge at times. The most important thing is to not show the child that it bothers you. Hard to help feeling that way, but easy to keep it from being the child's burden.