In discussing an individual as "the daughter of the only boy in my family," how would that person relate to the speaker?

The individual referred to as "the daughter of the only boy in my family" would relate to the speaker as a niece. This is because if the speaker has only one boy in their family, that boy is the speaker's son. Consequently, the daughter of that son would indeed be the speaker's niece. The familial relationship follows the traditional structure where the parent's sibling’s child is considered a niece or nephew.

The other choices do not accurately define the relationship: a granddaughter would imply that the speaker is a grandparent, a child would suggest that the individual in question is the speaker's own offspring, and a sister would indicate that the individual is a sibling of the speaker, neither of which aligns with the description given in the question.