Under normal circumstances, it's actually very difficult for a grandparent to get any kind of rights to see a grandchild. It's pretty unlikely that she'd be able to meet that burden, and unless and until she somehow manages that, you and your wife are the ones with full authority to decide whether he gets to see your kids at all, and what the circumstances are if she does.
She can sue for grand parents rights, but will have to hire an Attorney and file a petition first. The number one concern is the protection of your children. No Judge will grant access if she is violent and harmful. Especially if she views treating a disease with religion. You have the absolute right to set the time, place and conditions to see your children.
Providing general answers are meant to help the poster to understand some complex legal concepts and in no way creates an attorney-client relationship.
The law gives parents the right to determine who will have access to their children. That includes grandparents. You mother could file a lawsuit to get 'grandparents' rights, but grandparent rights are very limited. There is a U.S. Supreme Court ruling that limits grandparents rights and there is Texas law that follows the US Supreme Ct ruling. Your mother does not have 'rights' to see your children unless a court orders those rights. Just say NO!!
I recommend that you consult with an attorney regarding this matter.