co-uncle

of a man, an uncle of one's niece or nephew who is not one's brother; a co-brother-in-law as common uncle to the children of two men's married siblings.

Cicero continued however to write anxiously to Atticus, his co-uncle, about the young man's remarkable gifts and unsatisfactory character. [Cicero's younger brother married Atticus's sister, and the couple had a son Quintus; Cicero and Atticus were thus co-uncles to Quintus, the nephew they had in common]

a man's brother, as uncle in common to the children of a third sibling

My nephew came up for the summer with my brother and co-uncle George.