I think a lot of her behavior is age-related. I also think she may have some resentment problems. Is her BM in the picture?

Kids test boundaries. That's why someone invented Advil and alcohol They get mouthy, and it's up to their parents to correct it.

Is she in counseling? Is BF teaching her healthy ways to vent about the situation, like writing in a diary and/or talking to him in a respectful tone?

Perhaps you should suggest to BF that he enroll her in counseling and I think you should insist that he have her start treating you with civility, at least.

This whole "step" journey is hard on both stepparents and stepkids.

I agree with anon2009. Alot of my girl's moutiness started right at 8, and they are bio daughters lol. I found it helped to actually sit down with them and ask what was bothering them, then when I got the "i dunno" I would explain to them that sometimes they get cranky or feel sad right now for a reason they can't figure out and part of it is just changes in their bodies, but they still need to act respectfully to me and fiance. Don't know if that would help you out, but it seemed to help mine.

Well.. I would bring up to BD about her behavior and ask him to please be stern with her and tell her that it WILL NOT be tolerated anymore, that you are the adult and she will respect you PERIOD!
And if there are anymore issues that it is a PROMISE he will be handling it with her himself.
Yes some of it is age..but EVEN still..they know right from wrong, and it just will get worse as they get older, so get it nipped now while she is young, assert your position with the back up of your boyfriend.
You have to have be a united front,leave her no space to think she can come off to you just any kind of way.
" Life is like a jar or Jalapeno peppers, what you
do today could burn your a*s tomorrow."

You just described my life with Creature except she is only 5 and I get all the snotty attitude. Are you allowed to discipline her (ie, she gets mouthy, she goes to her damn bedroom and stays there?)? Personally, I am stuck with Creature after school for about 1 - 1 1/2 hours most days. I loathe it. I don't do anything for her either (I don't even do her laundry). The only thing I do is cook and only because I am cooking anyway (she eats what I make or she doesn't eat). Anyway, I used to have to watch her sometimes in the am before school (she goes to pm kindergarten), which meant I HAD to do things for her (get her breakfast, lunch, get her on the bus, etc.). I was pissed and one day I decided I wasn't doing it anymore. I am not her parent.

I talked to dh about the resentment I had toward her because of her attitude problem and because we paid her mother so much damn support and still had her here all the time. I told him that there were two issues with it 1) I didn't want to do it and was therefore resentful and ignored her which wasn't healthy for HER and 2) when children don't think they have to listen to you it becomes a safety issue because they'll spitefully do something you tell them not to do and I don't want that responsibility. Now Creature goes to her mother every morning. It definitely makes dh's life harder and I feel bad about that, but she isn't my kid. I didn't sleep with her (eeww, douche chill) mother and I have no obligation to baby-sit a child I don't like free of charge.

The key is to make it about the way SD treats you and the resulting feelings that make it difficult for you to be nice to her, which isn't healthy for either of you. Tell him that you've made the decision to stop doing things for her and if she is there ALL day, he is forcing you to go back on that decision (you can't let her starve) and that really upsets you and you'd prefer if he could find an alternative for those days.

"A pessimist complains about the wind, an optimist counts on the wind changing, a realist adjusts his sails"

There IS a difference between having a different opinion and being an asshole, find it.

LOL- I WAS going to type, " ask SVS", she is the pro and was my mentor for a long time...