You Can Work It Out
As they say in the old metaphor, "you catch more flies with honey than you do vinegar."
Being nice to people who are nice to you is actually pretty easy. But...being nice to people who are NOT nice to you takes some work but in the end it's a win, win situation and the way the world becomes a better place.
While we can't control the way others behave, especially the negative and angry people, we can control our own behavior and outlook. Beginning with not taking things personal. A good start is putting a smile on your face and letting it go.
So, when the parents alienate the grandparents for whatever reason, repeat the idiom in your head as a reminder of the virtues of being sweet when you want something.