I seem to keep having this problem with her. every 6 mos or so she gets ballsey and picks a fight like this. I try to stay out of it between she and Mrs NJ except for moral support and general bad mouthing, but there comes a time for me to step in. And I do and I flip my shit on her and everything settles down for a period of time.sometimes you have to fire a shot over the bow of the grandparents (to put them in their place) usually an out of town thanksgiving or xmas is a good FU to sent the grandparents..
You could also move to Colorado
Maybe Colorado is the next step...