There's a big difference between negative "people" and a negative mother. I can speak from experience! I always tell people that my mom didn't just see the glass half empty, she wondered who drank the other half of her glass.
I can only suggest two things. One is remembering that your mom sees the world in a funhouse mirror: no matter what angle she views it from, it will always be skewed and stretched.
The other is applying a quote from my therapist, about my mother: "She may have done the best job she could, but that doesn't mean she did a good job."
I'm glad you have your sisters for support, you can all be strong together.