What makes you so sure you're the one making mistakes? It is just as, or more likely to be your partner not parenting properly, or the SKIDs' bio mother spiking your guns. I'd recommend reading "Stepmonster" by Wednesday Martin, and also read this: http://www.stepfamily.org/dynamics.html

What kind of mistakes do you "think" you're making? Or is someone telling you you're making them??

Treat the kids like they are your own and you love them as much as possible. Don't trash talk the bio mom (even is she is a butt-ugly, scandalous, drawer-dropping whore.) Watch your back until you can be sure the mom isn't using the kids for her malevolent plans.