X and Z are sisters. Y is the brother of Z. S is the daughter of Z. T is the brother of S. T is married to U. How is Y related to S?
Mother’s brother
Z is S’s mother. Y is Z’s brother. Therefore Y is S’s mother’s brother, or maternal uncle.
A and B are brothers. C and D are sisters. A's son is D's brother. How is B related to C?
A is B's sister. C is B's mother. D is C's father. E is D's mother. Then, how is A related to D?