Step/blended families are a really tough gig. I've done it twice - on both sides of the fence. My ex brought two of his boys to live with us when we were together, and then, DD and I moved in with my current (and hopefully last!) DP when she was two.


When there are differences in parenting style it's hard to reconcile. And you're always stuck in the middle

I think the best course of action is some family counselling just to get the issues out in the open and resolved (or at least worked on) with some professional input.
There are also organisations for Stepfamilies which can give you some great advice on how to navigate the minefield.

Honestly, though, it's impossible to keep everyone happy without compromise. And, your DD, cant compromise her need for her. Your DP needs to be understanding that it's a massive period of adjustment for her. He is a grown man and he has to come second.