My husband and his brother, Tim were estranged--cut direct on both sides for about 7 years. The first time they saw eachother was at a mutual friend's funeral. Tim pointedly ignored my husband and my husband respected that while the Tim was there. Tim left early. My husband was uncharacteristically diplomatic when people asked about the awkwardness. "We don't speak. We'll get over it or we won't," and then he bean dipped like a champion.
The cut direct is so very awkward for bystanders, but if you need to hold your ground, hold it. Both Tim and my husband were there to show support to the family of the mutual friend and were able to do so. You won't be able to predict their behaviour, so manage your own. Keep a low profile at the service, pay your respects to the family of your departed friend and exit quietly. You don't have to engage.
If your cut family attempts to enage, you don't. They don't exist. Again, exit quietly. This is why it's the etiquett nuclear bomb. If you engage in any way, that is a peace overture.
The circumstances of my husband and Tim are obviously quite different and after that funeral, they have mended the fences between them. They have a closer relationship now than they did as children, to the shock of all.
(I would only confess this here on an anonymous internet forum... I only ever got my husband's side of the story and I think he was definitely the Wrong One in that dispute. I admire Tim for forgiving him. And my husband for finally admitting fault.)