The interview - whether for medical school, residency, fellowship, or dental school - is subject to basic legal rules. Admissions officers/faculty members should refrain from asking questions that are not relevant to the position the interviewee is seeking. Questions about race, religion, and marital/family status fall into this category.
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 if my home life will affect my ability to carry out my duties. I can assure it won't.")
If you have the opportunity to give feedback to the institution, you can consider doing so after the interview. When I was interviewing for residency, I was asked by a faculty member if I had a boyfriend. After the interview day, I talked to a faculty mentor at my school who reported the situation to the other institution. The faculty member who asked me the illegal question was no longer permitted to interview.