I used to work on these encephalitis at Ft. Collins CDC, in the early 80s. We had a human vaccine for west nile and both equines. However, it came with SERIOUS risk. 5% of the ppl would be put in a hospital. And 1%, give or take, would be mentally incapacitated (i.e. we KNEW that it would cause brain damage). Therefore it was limited to just those of us working on it. 
You MiL survived, but she was one of the very few that got the worst case of it. Most ppl actually do better than that. 
However, as somebody who has suffered from Vertigo/memory/processing issues, I feel for your MiL. It is not fun to lose capabilities like that. Worse, most ppl who have suffered the harder effects, not only tend to have the mental issues, but typically turn a bit more aggressive.   Hopefully, that is not the case for her.