it is probably hard for your mom to realize how bad her sister might be because she still looks at her through colored glasses of yester years while all you see is your mom's nut case sister.
...that's what I was getting at with my ramblings. People have a way of looking past the bad in their relatives. The ability grows with age and really blossoms when something bad happens to a relative.
The classic example is the the interview with the family member of a morally bankrupt multiply felonious piece of garbage who's just gotten his ticket punched by a cop. It's rare for them to have even a single negative thing to say about him.