The opposite relationship of uncle would be Niece or Nephew.
(The opposite gender is aunt.)
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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.
Assuming we are talking direct line, your uncle is a sibling to one of your parents. His uncle is also an uncle to one of your parents and therefore your grand uncle.
Your uncle is your father's or mother's brother so your uncle's father is your grandfather.
Your mother's uncle is your great uncle.
If he is your uncle by blood, his uncle is likely to be your great uncle. As to be an uncle it has to be a parents sibling. His parents would be your grandparents. So any of your grandparents siblings are typically called great uncles/aunts