The interview - whether for med school, residency, or fellowship - is subject to basic legal rules. Admissions officers/faculty members should refrain from asking questions that are irrelevant to the position the interviewee is seeking. Questions about race, religion, and marital/family status are no-nos.
When I was interviewing for residency, I was asked about my dating status... If you think these missteps are old news, a lovely client last year told me she was asked about her family planning. Very awkward.
If you are asked these types of questions, you can simply answer (if it's not distasteful to you) or respond by addressing the intent of the question without revealing personal information. ("I think you're asking whether I'd come to your institution if accepted. I can assure you no family issues will keep me from attending.") You can also refuse to answer the question; of course, this last tactic might cost you the position you are seeking - as unfair as that is.