It's too late for this family, but for others who may face similar questions from a child in the future, please remember the response proposed by Ann Landers or Dear Abby to such a question. When someone asks you a question you do not feel is appropriate to answer, respond with a question of your own: Why on earth would you ask me a question like that? Their response can then lead to a discussion of privacy, boundaries, etc. In an interpersonal interaction, the person who is asking the questions is in control of the interaction.
Best wishes to all.
Howie