If it's what your mother wants and you think there may be some arguing or tension between her and her grandchildren, or you feel that by them discussing religion around her it isn't what she wants and would create further tension then you are doing the right thing.
But your posts seem to come from the angle that it is about you and your rules, along with your feelings on the way you have been treated by them. I may have it wrong and its just the way i read it, but to me not allowing them in (if she wants it) is stooping to their level of conditional love for family, maybe its a way you're gaining the upper hand or power over them again. Personally i believe there is more power in being dignified toward them.
One of my greatest memories is seeing my grandmother in her last few days surrounded by family, many children and many, many grandchildren. There is a split in our family, not JW related, but a split nonetheless ... yet in front of her this split was put aside for her. It was different in another part of the house where the split was evident, but in front of her, in her room, we all knew it was her last days and wanted to spend our last moments with her. Many of those moments she was laughing.
I'm sorry if i come across as harsh, that isn't my intention and i'll not post anymore on the matter or this thread as its obviously a difficult time, I just think in moments like this the focus should be about her last days and her family, rather than the focus being about whats going on between yourselves.