Translation:An uncle is the brother of your father or your mother.
Masculine nouns beginning with z or s+consonant use uno instead of un. See here http://www.bbc.co.uk/languages/italian/language_notes/un.html
Probably, it's just because in English is "of your father or your mother".
French people would also say" L'oncle est le frère de ton père et de ta mère" Lo zio è il fratello di tuo padre e di tua madre.
Lo zio è il fratello di tuo padre o di tua madre. This is the way an Italian would speak
I was marked wrong for leaving out the second 'your'. I think including it makes for wery clunky English which sounds unnatural.
Wouldn't the husband of the sister of your father or mother also be your uncle?