I feel your pain. My son, who I adopted 7.5 years ago, exhibited extremely similar behaviors for several years and it was beyond frustrating. We were also given the diagnoses of ADHD and ODD. What I later discovered was that these were just symptoms of a much larger problem of reactive attachment disorder (RAD). Kids with RAD missed out on some critical brain wiring stages in the first three years of life due to traumatic experiences – these are typically abuse or neglect, but can also be associated with an early life spent in the hospital with a medical condition, such as prematurity or another problem that involved the baby being exposed to a lot of pain. One thing that helps these kids a lot is neurofeedback. It has very much helped my son. I would recommend checking out the website for the Institute for Attachment and Child Development (IACD) to see if the articles that are written there ring true for you (http://instituteforattachment.ong). They use neurofeedback extensively to help traumatized children’s brains rewire, which calms their anxiety and helps them to begin to control their impulses.
One thing that’s really important for you to understand is that you may be becoming a victim of Post Traumatic Stress Disorder yourself due to the difficulties of dealing with this situation. IACD has articles about the “nurturing enemy” syndrome that might be helpful for you to read. Unfortunately, my experience has been that most therapists don’t have a clue about RAD, and often give poor advice. The articles at IACD may help you to feel validated about what’s been going on in your household and may offer some ideas that could help you to move forward.
Know that your step-daughter is lucky to have someone as caring as you in her life. All the best.