Questions: Cuando eras niña, construías casas con tu hermano?

The answer is the first one: Sí, yo construía casas con mi hermano.

Explanation:

• "Sí, yo construía casas con mi hermano." is the correct response because it directly answers the question in the first person, matching the subject "yo" (I) with the verb "construía" (I used to build), indicating that the speaker used to build houses with their brother.
• "No, ella no construía casas con tu hermano." is incorrect because it changes the subject to "ella" (she), which does not match the question's subject "yo" (I).
• "Sí, construía casas con tu hermano." is grammatically correct and matches the question, but it lacks the subject "yo" for clarity, making the first option more complete.