Ah, siblings. It can be the best or the worst. I used to watch my stepchildren fight, doing and saying terrible things to each other, and wonder how on earth they could reconcile after all that, but they always seemed to, eventually.

if two adults want to fight, there's nothing you can do, EXCEPT listen. You and your parents can't side with either of them because it's all too likely the one 'picked' as right will report that to the other. "Mom and Dad and Sis think you should apologize." Then the other is fighting with everybody.

You can validate their feelings but be careful not to agree with their positionin the argument. "I can see why you felt angry over that." But don't say, "No wonder you're mad! That was a lousy thing to say to you."

If your siblings are agreeable and it's geographically possible, invite the grandkids to family gatherings. "You know you are always welcomed and wanted here, but if you think you should stay away, could you drop the kids off? Or somebody can pick them up and bring them back."

Nobody talks to the kids about the fight.

You will not tolerate your siblings not talking?

This has nothing to do with you.

I’m sorry, that sucks