Welcome to A2K!
I gather Mommy is still alive and living with the other grandparents? If so, no matter how unfair it may seem to you, I'd have to recommend that for the children's sake that you, as the adults, make the adjustment and avoid the bickering to the greatest extent possible. If it is indeed child abuse, you are unfortunately inadvertently complicit in it, if you’re complicit in maintaining it as an issue when there's likely nothing you can do about it. Best wishes, and sorry to hear about your loss.