The school counselor or your pediatrician can refer you to a therapist in your community who can help with these behaviors. You will want to find someone who will work with your entire family, as you will need to work together to improve the problems that you are experiencing.
Since no one else has seen these extreme behaviors, perhaps it is only when she is with family members that your stepdaughter exhibits them. The school staff apparently has seen the problems she is having with achievement since she is going to repeat kindergarten. Be sure to let the teacher and school counselor know that you are pursuing therapy for your family.