To begin with: Name it. While it could be embarrassing and problematic for stepparents to acknowledge (to by themselves, let alone aloud to other people) which they could be experiencing jealous of these partner’s kiddies, acknowledging you are experiencing jealous before it evolves into whatever else, may be the first rung on the ladder in conquering it.
Next: When you find that you're feeling jealous, have moment, inhale slowly, observe your thinking and emotions.
Be truthful with your self. Does it stem from being within an unknown spot; from feeling omitted, excluded and powerless when your partner is parenting and taking care of her kids? Will it be because, if your step-children are about, you're feeling than you are like you are the last one on your partner’s priority list, that your needs come last and that the kids are much more important to him/her? Does it reflect that seeing your spouse making use of their children provides you with a definite image of an as soon as pleased family members which he was part of and you also are not?