First of all, my condolences. That is a terrible, terrible thing to have happen. I feel especially bad for your sister-in-law, as this is pretty much a worst-case scenario -- conscious and aware, but unable to do anything. The mere thought of being in that kind of state terrifies me.
The brain is quite resilient. Your idea of some sort of brainwave device may actually have some merit; the "biofeedback" craze of the 1970s and '80s demonstrated that you can train yourself to modify your own brainwaves (and other "involuntary" bodily functions), and people have been working on brainwave-based control devices ever since. I'm not sure what's currently out there, but perhaps a creative combination of off-the-shelf sensors and some hacked-together interfaces to a laptop or Raspberry Pi type device could yield some useful results.
If you don't mind telling, what is her prognosis for recovery? Is this believed to be a temporary, or (shudder) long-term/permanent condition? This will certainly affect how you will want to proceed.