What is your uncle's brother to you?

It depends on how he's your uncle.

If he's your uncle because he's your father's brother, his brother is also your uncle. Or your father.

If he's your uncle because he's your mother's brother, his brother is also your uncle.

If he's your uncle because he's married to your mother's or father's sister, his brother is pretty much "no relation" to you, or at least English does not have a specific phrase for the relationship that's more concise than "uncle's brother".