@Tisme,
Make sure your eldest knows you have their back and are on their side.
My paternal grandmother was foul, and my father always took her side regardless.
I get on very well with my kids, they know I am always on their side in any dispute they may have with my father.
You can't change your mother, but you can do your best to be nothing like her.
Focus on your children, they're the ones who matter.