How is one individual connected when another states they are “the only child of my mother’s sister”?

When one individual states they are "the only child of my mother's sister," they are referring to their maternal aunt (the mother's sister) who has only one child. That child is the speaker's cousin. In family terms, a cousin is the child of one's aunt or uncle, which precisely matches the definition given in the statement.

The other choices do not accurately describe the relationship. A grandchild would refer to the child of someone's own child, a nephew is the son of one's sibling, and a brother is a sibling to the individual, which does not apply in this context. Thus, the correct answer reflects the family connection being described.