Just had a good friend of mine around who has recently become a stepmum as well as having her own DC's. And like my wife & I, she's also struggling to integrate the family as a unit.
It's just something a lot of our friends who have never been a step-parent understand. They all assume that as soon as you live together that it's all happy families. It's usually a surprise when I say that in many ways it was easier being a single parent. There's no compromises. You don't get put into impossible situations where you are damned if you do, damned if you don't.