Anupam said to a lady, “The only daughter of the brother of my wife is the sister of your husband.” How is Anupam related to the husband of the lady?

The only daughter of the brother of Anupam's wife is Anupam's wife's niece. She is the sister of the lady's husband, so the husband is the son of Anupam's wife's brother. Anupam is therefore the husband of the husband's paternal aunt, i.e. his uncle (paternal aunt's husband). He is not a maternal uncle (mother's brother), father or son-in-law.