Truly sorry to hear about the condition of your sister-in-law. It is a very difficult and sad situation to be in. I'm guessing she is mostly taken care of by your mom-in-law.
There have been a couple of similar cases on here which were refused TRVs.
Your sister-in-law is 100% dependent on your in-laws for her every necessity and her well-being. Added to this traumatic situation is the fact that she cannot communicate nor walk (even with support). Quite honestly, IMO, there are absolutely zero chances of her getting a TRV