***Rather than imitate the Society's lack of love, why not reason with the family to show support for all, especially your sister, who must need some attention. Perhaps your reasonableness could influence your mom.***
Quandry, your suggestion is, of course, the best option if all parties agree to be reasonable.
Unfortunately, JWs can be very unreasonable in their absolute certainty of belief. If JW grandparents aggressively ignore the parents' warnings about pushing JW doctrine onto innocent, impressionable children, then the children's welfare MUST COME FIRST.
Being told contradictory things by parents and grandparents and being pushed into a religious tug-of-war is not in a child's best interest.
If grandparents refuse to back down, the parents must, for their children's sakes, take whatever steps are necessary to limit the grandparents' unhealthy influence. It's unfortunate, but sometimes necessary.