Co-uncle

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]

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