Hi! I am experiencing something similar (but not as extensive as you have) from a woman at my job who everyone respects and another manager - a friend of hers.
The two are so vile, they even have nicknames for alot of us they look down upon. My nickname is "Velmo" --- supposedly a dorky verson of Velma from the old Scooby Doo cartoon. Mind you, these two "adults" are in their 40's and 50's but behave like 8 year olds.
What I do is just ignore them, and dont give them the reaction they are looking for.
In your situation, if someone in your family refers to your mother you can use that opportunity to praise your mother in some way behind her back. While she is defaming you, you can prove her a liar to others by actually doing the opposite (but dont go overboard!)
Then when your Mom gossips again, they will be able to think for themselves and realize she may not be playing with a 'full deck"