Sentence:  Cicero continued however to write anxiously to Atticus, his <WSD>co-uncle</WSD>, 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] 
Meaning of the word marked with <WSD> tags:  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.