Sindhu introduces a man as the son of the brother of her mother. How is the man related to Sindhu?

Let's break down the relation step-by-step:
1. "Her mother" $\to$ Sindhu's mother.
2. "Brother of her mother" $\to$ Mother's brother is Sindhu's maternal uncle (Mama).
3. "Son of the brother of her mother" $\to$ Son of Sindhu's maternal uncle.

The son of one's uncle is their cousin. Therefore, the man is Sindhu's cousin.