X and Y are children of A. A is the father of X and Y is not the son of A. What is Y to A?

The problem states that A is the father of both X and Y, making them siblings. Y is 'not the son of A' - this eliminates the possibility of Y being a brother. Since Y is A's child but not a son, Y must be A's daughter. This is a classic logical reasoning puzzle that tests the ability to process negative information and deduce the only remaining possibility from given constraints.