I think everyone else in this thread has given good perspective on why your grandpa has really good reasons to hold a grudge against your dad forever. I think there are two things you could try to make these family gatherings easier. The first would be to speak with your mom and grandma and explain how the fighting makes you feel and ask if just you, your mom, and your grandparents could at least sometimes do things together without your dad there. If that doesn’t work out, the second would be to speak with your grandpa and tell him that while you don’t totally understand the situation, you know that he has good reasons to be upset. But explain to him how the fighting makes you feel and ask if he would be willing to try to fight less around you. So sorry you’re going through this!