She is most likely miserable with her life and takes it out on you and your sisters. Other than you and your sisters going and all telling her together that you don't appreciate how she puts each one of you down to the other I think you just have to accept that this is the way she is.
Talking about others and doing the things she is doing and then playing the victim is passive/aggressive behavior. Usually people that use passive/aggressive tactics aren't aware that they do this but they seem to have it down pat because it is a behavior pattern they have gotten away with for so many years. Over the years people leave it alone because they don't see the harm they think "Thats just mom for you" But as they develop the characteristics they become worse and by the time you realize it it is too late to change them because then they are
To the point they cry victim because they see it as you attacking their character.
Passive-Aggressive (Negativistic) Personality: People with a passive-aggressive personality behave in ways that appear inept or passive. However, these behaviors are actually ways to avoid responsibility or to control or punish others. People with a passive-aggressive personality often procrastinate, perform tasks inefficiently, or claim an implausible disability. Frequently, they agree to perform tasks they do not want to perform and then subtly undermine completion of the tasks. Such behavior usually enables them to deny or conceal hostility or disagreements and get the desired results they want making it a no win situation for others.
These are some things you need to keep reminding yourself
"This is their problem, I will not make it mine."
"I'm not going to allow anyone to dictate my feelings."
"They want me to fight with them, I won't let myself get sucked into it."
"Their need to be difficult is a cover-up for their own inadequacies."
"I have the choice to play or not this game."
This book is dealing with the passive/aggressive man but I am sure the scenerio is the same.
Amazon.com: Living with the Passive-Aggressive Man: Books: Scott Wetzler