How do you get a grandparent to stop undermining you as a parent?

We made a huge mistake and decided to move right across the street from my parents. Now everything would be fine if my parents weren't raising my sisters kids. But since they have raised them as grandchildren and not as there own the have become monsters. Our parental style involves love,respect, and discipline(nothing extreme but they get they get the point.) Well of coarse the cousins have issues with each other and just don't get along. My nieces are manipulative and just plain mean. The whole neighborhood can't stand them at their houses and none of the kids want to play with them. I brought this to my parents attention and they have just given up. We are moving in the Summer but now they try to find anything and everything to pick on about my family/kids/their other grandchildren. I has really becomes a huge problem and nasty words are being exchanged. I feel our family ties are coming undone and I'm not going to let my kids suffer the abuse by their cousins(verbally). After trying to explain a simple rule of buckle your seat belt or you don't ride in my car. I was then told I have an attitude and that my nieces are now to ignore me. Mature right? So now I don't take them anywhere. I can't avoid them because they are so close but I really want nothing to do with them until they realize they need to address these children's behavior. What should I do?