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The brother of your aunt's husband is not your uncle.
Also the brother of your uncle who is not your uncle is your father.
yes ! but not your uncle in text as such....
Your mother's brother is your uncle.
My uncle? Or uncles in general? Or your uncle? My uncle is genial. Uncles in general are avuncular. Your uncle is unidentified.
The singular possessive form of "uncle" is "uncle's."