I'm also a stepmom whose husband has primary custody of his kids, so I get where you're coming from. I think that while your husband is the heavy, there's no reason you can't back him/your household up on things like this.
It's hard when they say "why?" and you don't want to disparage Mom or Dad. We had similar issues. When my SD came back knowing ALL the lyrics to Kid Rock songs we cringed. When she popped Goldmember in the DVD player at 8 yrs old, I sat down to watch because her friends were coming over and I didn't want to not know what the movie was about. Then I said, "SD, I can't let you watch this with your friends. It goes against our rules. You should take this back to your mom's house." Or when then-7 yr old SD wanted to walk to the store like she did at her mom's place and we wouldn't let her, and her mom called up DH all ticked off. He said, "We don't feel it's safe for SD to go to stores by herself. We will not budge and we hope that you take SD's safety into consideration when she's with you."
Especially if there are other kids in the house, having "house rules" can simply be "I'm sorry, but these are our house rules. That CD needs to stay in your bag til you go back to your mom's house."
Kids do get that they can do x at one place and y at another. Schools, friends' parents, grandparents, camps...they are all places with different rules. Sometimes it helps to point out these things so that they have a point of reference. "You know how you can't chew gum in class? Well, same here. Different places have different rules." Try to treat it as if she did a sleepover or went to a friend's house and they allowed something you don't.
I think that she's starting to notice the differences more as well as getting to an age where she wants to DO more. It can be tough to be the "lame one".
For things like bedtime, enlist her. When SD complained to her mom that she had a "baby bedtime" we said, "Okay, what do YOU think is appropriate and why?" and we worked out an agreement where she could earn more time by doing what she was supposed to for 2 weeks. If she didn't make us late AND got to bed on time, she earned 30 minutes. It took her a LONG time to get to 9:30 and all we had to do was point to the agreement. It took the battle out of it.
Hope that helps.