I think that if you fight this battle too hard, you are likely to lose. Play it cool, and in time she will begin to challenge her stepmother and be more willing to listen to you. Right now she’s playing each of you against the other. Chances are, when she is with her stepmother, she isn’t as accepting of the way things are done there as she sounds when she tells you about it. If you do something and she informs you, “That’s not the way _____does that,” come back with something like, “Well, tell me how she does it, and I’ll think about whether it would work here.” Give in on little things. If the difference relates to something really important, stand your ground.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.