Redhot: you'll have to put your foot down to make them stop.
When my son was little my mom went so far as to show up at his school (without my knowledge) and tell his teacher that he didn't celebrate holidays . Thank goodness the teacher called me to confirm and I was able to set the record straight!
I was scared to be tough about it for a long time because I didn't want to loose those few precious relationships I had - but in the end, I realized that my children were much more important. It took some serious talks with my parents (and they didn't like what I had to say) - but they finally backed off. It was either that - or they could not see their grandson any longer.