I thought labs were supposed to be THE perfect family friendly dog??? Everyone who owns one brags about how they are so loving and protective of their families, especially the children. So I wonder what details are missing....
I have had my own personal dog around children opinion completely reversed several years ago. My entire life, whenever I heard of a dog just nipping at a child, in the back of my mind I always thought for sure the child did something to provoke the animal. Kids are just that way naturally with their erratic physical behavior, it wouldn't take much to set off a skittish dog.
Well, we had a mixed breed rescue dog, PAWS lied to us about the age to make him more adoptable. We were camping with my family, I was laying down, having my pregnant afternoon rest, dog at the foot of the bed, when my niece walked in and did not know I was there, she gently and quietly came up along the side of the dog and placed her little hand on his back, between the shoulder blades. Nothing alarming. The dog turned on her and attacked her. And when he didn't get a good bite the first time, he went after her again. I can't even imagine what the outcome would have been had I not been right there watching. She was less than 1/8th of an inch away from losing her eye. We put that aggressive dog down and I've been pissed at PAWS ever since.
So without knowing the complete history...was this an isolated event? Or is it a pattern? How attached is your mom to the dog? How old is the dog? Does your sister blow things out of proportion a lot? Or seldom? Was your sister just waiting for an opportunity to set a boundary with your mom over the dog? Can a more common sense approach be taken, like kenneling the dog when kids are around? Or would mom do it her own way when the adults left? Seems like there is a lack of trust between the two already, and this just set it off.