Have I got this right please?

[Olde Crone]

Two male first cousins both born in 1753 marry two sisters. The males also share one great grandmother. The two sisters are the daughters of first cousins and everyone shares two sets of great great grandparents!

My argument is that the children of the men born in 1753 will have a genetic relationship that MAY make them all look as if they are siblings or half siblings rather than first cousins because their genetic pool is so small and concentrated. I have been contacted by someone who has had a dna ancestry test and is adamant that he and two other contacts all descend from one couple, whereas I think there is room for doubt.

OC