You should never support nor accept backbiting because it is a major sin, but on the same time you must keep the respect of your mother and avoid hurting her feelings. Keeping these two duties needs your wisdom. Stopping her from backbiting can come from making her realize that by backbiting those whom she's not like, she will loose her good deeds which will go  to them, and might also get the burden of their bad deeds. You should be very polite in talking to your mother and never look like trying to teach her. You can tell her stories and narrations from the Prophet (SAWA) and Ahlul Bayt (AS). 

Wassalam.