I'm with the others. This is a documentation of family history not a squabble between warring factions.
I think there are so many families who don't have good photos of all their members together it borders on criminal.
One's identity is intimately tied up in who his relatives are. All your aunts and uncles and grandparents, your brothers and sisters, your cousins and your mother and father are all the people who made you who you are. It's a shame when we don't have pictures of them to see after they are gone.
My personal belief is that people should be able to suck it up and swallow their pride long enough to sit down for a family portrait. They can sit on opposite sides if they want to.
As the others say, it's not your responsibility to be the go-between. You should do the project the way you see fit and, if others can't play nice long enough to sit still for a picture that's their problem, not yours.
Maybe you can get photos of the different people separately and include them in the album as individual photos.
Talk it over with your uncle and the others. That's the best course of action. But I would gently make it known that if somebody wants to be left out of the family album it will be their own loss.
I wouldn't want to be the one missing from the family album when future generations of kids ask, "Who was Uncle Randy?"