My SD isn't a very nice girl.
I'm sorry this is happening to you. I don't have alot of advice on what to do, only offer up some empathy since I have that same 13 yr SD.
She lies, she hates me and slanders me all over facebook ( her dad finally had her delete the FB account, why should immature young children have such a public forum and not know what trouble they are really causing?) All I have done is take these kids in when they wanted to get away from their severley alcoholic BM, who endangered their lives on a daily basis, feed them, house them, love them, provide for them etc. all to be treated like a second class citizen in my own home now. I feel like a fool ever having the gumption to care in the first place now. This is what it got me, kicked in the teeth and to some point, my DH does not take enough control and gave it to her.
It is no longer a happy household ( I guess when *I* thought it was , I was under false impressions), I dread coming home after work now, I work longer hours to avoid it. I have a sinking feeling in my stomach everyday and my souls feels sick. I no longer wish to bake yummy treats, cook and collect recipes or clean. Her attitude towards me is crushing my shoulders.
I'm not saying what your SD did was right, or even BM.
But please try to remember that your SD is only 13 and at a vulnerable age. When her dad meets someone new it's hard because all of a sudden, this new woman can take some of your daddy-and-me time away.
Every child is different, so this is only a specualtion.
You may be a threat to her time with dad, at least how she sees it. I have no idea if BM is forcing her to do anything, or reinforcing ideas or what have you.. but try not to take it TOO personally. She may be under a lot of stress, especially at her age if things are changing and going wrong from her perspective. Some kids will do almost anything just to get those good times back.
A lot of kids will use manipulation tactics to spend more time with their parents if they hardly see them. You say your husband works 2 FT jobs.. and then from step daughter's POV, what little time he has in between he's spending it with you and not with her. I'm not saying you or your husband are in the wrong. Only how step daughter may see it.
Perhaps she could see a counsellor and talk to a neutral party about the whole thing or she and her dad have a sit down and a heart to heart. Perhaps you could too. But not before Dad does.
There's something not right here and Dad does need to get to the bottom of it.But please try not to hold it against her until you get the full picture. She will pick up on your feelings towards her and if you're angry it won't help the situation.
Ask me anything :)
What do you hope will happen if you tell the girl you know what you know?