Baby steps.
How about a goal of "Do your best"." Lets see how much you can bring your grades up."
Our daughter always made good grades., but we never put grade pressure on her. We always told her we would help her in any way possible to do her best. If she got a low score, we would always ask, "Did you do your best?" If she said yes, we told her we knew she was disappointed, so what did she think would help for next time?
If she said, well I should have studied harder, we asked "so how can we help you with that.?". She was in control of making it happen, but knew we were there to support her.
For step daughter, I suggest you meet with all of her teachers and the school counselor.. Remember school counselors are mainly "academic counselors".. not therapist.. They will be able to guide you as to what daughter needs to work on this summer. I
If she can attend a summer school program, see if they will allow her to take one subject. This will ease her into a homework schedule and allow all of you to see what she knows and does not know.
Reading may be a huge weakness for her, so have her read to you , and you and your husband sometimes read to her. If you have problems with her listening to you guys, she may do better with a tutor. Like a high school student or college student.. It will give you a and her a break.
Do also have her see a therapist and I suggest you also set up family therapist. This poor dear has been neglected and needs to be able to share this in a safe place.
Please have patience. This is going to take a lot of time. She will thrive with rules that are enforced and consider for some of the rules she can help decide the rules themselves and consequences.. Once again, this will give her some power and some ownership.
Put aside any preconceived ideas you have and let this all happen organically and all about this young girl. SHE needs the help from you guys,. Not more stress and pressure.