My almost 7 YO daughter is also a thumb sucker. She has OCD and sensory issues (read The Out of Sync Child, as it may be your daughter has sensory issues as well.) She also has Asperger Syndrome. It's very common for children on the autism spectrum to be diagnosed with OCD, ADD, bipolar, anxiety, or other problems but miss AS alltogether.
I am a tongue sucker. When I was forced to stop (yay dentists and their torture devices) I became a teeth grinder. That messes up your jaw, teeth and may cause other issues that are much more severe the sucking your tongue. So when I am stressed and notice I am grinding my teeth I allow myself to suck my tongue. Maybe you can find an alternative that will help her switch from picking to something else that gives her the same calming stimulation that she may be seeking in the picking (and prior to that her thumb sucking.)
Does she see a psychiatrist for her medication or just her ped? It's been my experience with OBs and pediatricians that they are not experienced enough to properly diagnose psychiatric conditions. I took my daughter to Dr Stokan in Stafford as I felt that she needed more help then her pedi was able to give. She has a therapist who works with her who is supposed to be wonderful with OCD. You are welcome to PM me if interested in her contact info. Also, your daughter may need an anti anxiety medication or risperdall instead of Vyvanse. This is something good to talk to a psychiatrist about. Stress and anxiety makes tics worse (new house/town, a step sister and a new baby coming are all typical anxiety inducers!)
No amount of yelling, punishing, bribing ect is going to curb this behavior as she likely feels a compulsion to do it and cannot control it. Think of it as a tic like Tourette's Syndrome.
J., I just realized that you are the lady with a step daughter with AS. So this is not entirely new to you. If they are blood related then it is even more likely that she does have AS. I have at least three on the spectrum (I'm unsure if my 2 YO has autism yet.)
Wishing you the best,
S.