2 options:
Option 1. Set some rules in place so she AND wifey understand.
Example:
(To Wife) "Look now, i really don't mean to be inconsiderate, but im gonna need mom in law to get it together".
(to Mom in law) "You are not that down bad to where cleaning your room is difficult. Please, respect my house and clean up after yourself. i love you, but being ill does not release you of being considerate of others. This is the living area, to be shared by EVERYONE. You are welcome to come in to relax, but we have furnished a nice area for you to have to yourself. i please ask that you would enjoy that area instead".
Option 2. Convert the downstairs into your office. If she finds a way to fukk with you there, then there is always the nursing home.