Who is you uncle's brother's cousin's sister?

Your uncle's brother's cousin is also your uncle's cousin. And the cousin's sister is also his cousin. So the question is the same as "Who is your uncle's cousin?"

If your uncle is the brother of one of your parents, then his first cousin is your first cousin, once removed. Similarly, his second cousin is your second cousin, once removed, and so forth.

If your uncle is the husband of the sister of one of your parents, then his cousin is not related to you.