Pointing towards a girl, Vishakha said, “She is my brother’s wife’s mother-in-law’s son’s daughter.” How is that girl’s father related to Vishakha?

Let's break down the statement made by Vishakha step by step:
"my brother's wife" → Vishakha's sister-in-law.

"my brother's wife's mother-in-law" → The mother-in-law of Vishakha's brother's wife is Vishakha's own mother.

"mother-in-law's son" → Vishakha's mother's son, which means Vishakha's brother.

"son's daughter" → Vishakha's brother's daughter.

So, the girl is Vishakha's brother's daughter.
The question asks: "How is that girl's father related to Vishakha?"
The father of that girl is Vishakha's brother.