Poor little girl, what an awful mother, try not to be scared, you will get used to it, do you have any family that can baby sit for you? Communication is the key, work as a team and try to make time for yourselves, bio parents need time to themselves as well, you havn't mentioned what your sd's behaviour is like, I expect that whatever her behaviour is like now, she will settle down now, without the stress of having to see her mother, maybe this will be the kick up the arse that bm needs? Im sure that if you talk to each other and work together this will work for you, thank god your sd has you and her father in her life to take proper care of her.

It's only 7 years until she's out of high school and presumably on to college or career flipping burgers.

Meanwhile you quietly disengage - especially since she is entering adolescense which will bring on rebellion against authority figures. Since you won't be one - if you disengage - you might actually end up being her only adult friend for a few years. Either friend or neutral 'roommate' your marriage should not suffer. You see when you're disengaged you don't join the fight over obedience and discipline you simply back up what Daddy says without actively participating in the discussion or rules, discipline or reporting of infractions.

I wrote this a long time ago and I hope this helps with your problem. It may not reflect your exact situation but you'll get the idea:
The situation with my step-kids finally got to the point where I decided that my efforts to raise them was futile.
First I told my wife in private what I was going to do and why.
Then I stopped investing myself, time and money in their upbringing. I realized that by conscious choice I could cease being responsible for them or their actions. Neither would I take credit for how they turned out, good or bad. I simply stopped interfering in their lives. This is not to say if they would start a fire on the living room floor I would not intervene. But if they did not do the laundry as scheduled I ignored it. Nor did I do any of their chores. If the trash overflowed in the kitchen - well tough. Mom dealt with it when she got home.
You will be absolutely amazed at the look on a kids face when s/he realizes you don't care enough to even tell their bio-parent they did something wrong. And you'll be amazed at their future behavoir and attitude towards you.
I stayed friendly enough and taught one how to drive when she was old enough. But they could no longer blame me for the consequences of misbehaving just because I saw the misbehavoir. I never told. If they were caught in a infraction it was not my fault. They began to realize that they were responsible for their actions because of what they did not because I caught them.
I would talk to my wife in private if I had issues, but once she made the decision I backed her up 100%.
Once the kids realized I was no longer the ogre they thought I was their attitude changed. I was not their friend, but neither was I the enemy. They came to realize that I had not done those things to irritate them. They in fact missed the things that they had come to depend on me doing. Permission to go to the mall when their Mom was not home? "Sorry, can't do that".
Their mother slowly came to realize that I wasn't overreacting to their actions. In fact I wasn't reacting at all. She came to understand that she would have to control the situation and she did.
With that things got much easier around the house.
Billions of kids grew up in the world without help from me and turned out just fine.