I'm with Marilyn on this one. I would be VERY careful what type of contact I would allow my children to have with the MIL. Tammie stated that she was going to "sneak" the children to the assembly. That implies that she's fully aware this would be in total defiance of the parents' wishes, and yet she would do it anyway. Since she's that willing to lie and manipulate, I would not allow her to be alone with the kids, as there is no telling what she might say to them. This is a case where the parents really have to stick to their guns and protect their children from harmful influences, IMO.
The MIL has the opportunity to see her grandchildren, she just doesn't like the rules her son and DIL have made...tough bunnies. It's her choice and they (the parents) have every right to set and enforce those limits. I do agree David that kindness is what is needed; however, I don't think that kowtowing to her religion is the way to go.
Dana