T.,
This sounds short, and looks short, but really, I am up against the end of a nap. So know I am thinking of you and hoping you are able to sort this out.
I am assuming you're married. If all you ever argue over is your step daughter - be thankful.
You will have to be the MOM of two beautiful boys and be patient, decisive and a few other adjectives that describe a MOM, and LET GO of your step-daughter's grades. You will also have to teach your sons right from wrong. Learn how to do so with tact and finesse, and you will have two sons who will know how to handle themselves in the future.
Just let her go. You cannot control her. You can't make her WANT to get good grades, please her dad, or anything else. I'm not telling you to not stress the importance of grades in society, just don't yell at her to get better ones. If she has been failing, it isn't going to come up over night - and she may be overwhelmed by the task of not just doing better, but erasing the past. Which is impossible.
You CAN, however, help open her eyes.
Let her see what good grades can get you - and bad grades too.
Find out why she is not pushing herself, ask to be included in on therapy (if you can).
Maybe she wants to live with her Mom, and doesn't know it or won't tell.
Maybe a lot of things.
I wish you luck, and hope you get a lot more ideas that work for you. I know this works, but it takes a lot of energy on your part.
Oh, and you are going to have to start being the other half of a couple. You include his daughter in your family and she is under your roof. You are her Mom away from Mom. She needs something - and finding out what it is is going to be rough, unless you take this situation with grace and run with it.
prayers for your daughter, and hope you let me know how things go,
M.