My 91 year old mother sounds like your mother-in-law. She was a good driver in her younger days. My Dad died 3 years ago so she is alone. We don't know how good her vision is but she has told us that she has macular degeneration in my eye. Same issue as yoru mother-in-law with the hearing problem. My Mom is very frail and can not turn her head around to see what is behind her. I have 2 sisters and we are divided on how to handle this mess. I'm thinking og talking with her doctor and get him involved. Have you tried this?