I think the suggestion that your mom visit you at your home is a great idea. They are really not old enough to be left alone with her if she is going to continue to talk to them about religion when you have asked her not too.
My mother very rarely let me sleep over my grandma's house on weekends because she went to church every Sunday. One Sunday I went to the church including the Sunday school. My mom was not too happy...until I told her that I kept raising questions about what they were teaching hahah. I was a devote little kid.
My sister told me a little story a few months ago. Her husband (nonJW) had asked their 2 year old daughter if she wanted to have a birthday cake. She was in her no stage, so of course yelled "NO!" Her dad laughed and said "Why not?" Then his 6 year old son piped up and said "Maybe she wants to obey Jehovah." My sister thought this was great. I probably would have 4 or 5 years ago too. Now, though, I think it is just sad that this religion is starting to put a wall up between a parent and his kids. It may be a little thing, but it will grow over time.
I hope your mother comes around to respect your wishes. It is a shame that her loyalty to the borg is greater than her love for her family.