I really agree with what others have written so far.
To add my two cents: We have a similar situation, where my dsd was 11 when my dd was born. She not only comes from a totally different type of household, but has a personality quite different from my dh's and mine. She's really loud, assertive, high-energy, while my dh and both tend to be pretty quiet and mellow. She lived with us full-time last year (when she was 13-14 and my dd was 2-3), then went home to her mom's for 5 months, and is now back full-time with us.
Firstly, I really agree with what others have said about how kids know very quickly what is okay at one house and not at another. They're quite smart that way, and the clearer you make your rules and expectations, the easier it is for them to adapt. Of course, you'll want to push your spouse to help you establish those rules, since it is easier for kids to accept when it comes from their parents. (I always try to keep in mind that I am not, and should not be, the primary disciplinarian.)
Second, as far as having control over the environment or conditions that your children are raised in, I really identify. Through no real fault of her own, my dsd has developed a lot of habits/behaviors that I really don't want my dd picking up. I was pretty worried that she would have a real impact in shaping my dd's behavior. And I suppose there's no way to avoid her having some impact on my dd, but I think it's important to let go of the idea of "controlling" your children's upbringing--as much as possible. If you can provide a basic foundation of love, support, structure, etc. for your children, they develop personalities and values culling from a broad range of influences.
I was interested to see whether my dsd's absence would have any impact on my dd's behavior when she returned to her mom's last August. I found that it really didn't. Specifically, my dd is pretty high-energy, gets frustrated easily, needs a fair amount of attention and interaction, etc. She's very much like her sister in that respect. So I wondered if she would mellow or have an easier time when it was just my dd and us. Not a bit. I really couldn't see any differences in her.
It reinforced for me the importance of something I had already been trying to do, which was resist the temptation to scapegoat my dsd for my dd's behavior. It would be a really unfair thing for me or my dh to do, and her absence for five months helped prove that.
I also think, in writing this, that it's important to think about the good things your stepchild(ren) can bring out in their sibling(s). My dd really, really loves her sister, and clearly enjoys having her back in our house. I love to think about how she will be able to confide in her as they both get older, and "escape" to her house when she needs a little break from us. I'm really glad that they both have an additional source of love and support in one another. It's important for us to help them cultivate their relationship.