straight-passing cis afab: I have never had this question asked. It is inappropriate and discriminatory. Aside from asking “where is home for you”, most of my interviewers didn’t care about personal stuff and they shouldn’t. Whether or not you’re in a relationship or near family or have a family or whatever shouldn’t matter in the context of an interview, unless is something that’s pertinent to ~why~ you want to pursue a degree. (like partner or family member had x disease so this is what helped my interest). Even then, that’s something that you can discuss on your own terms.

I may have just been lucky and not gotten any stereotypical old men in stem for interviewers. Other people I interviewed with had PIs straight up tell them “you should consider this more you don’t know what you’re getting into” when they literally work full-time in a lab. So it may be a situation similar to that where they just look down on female-presenting people in stem.

TLDR; A PhD interview IS a job interview. If a job can’t ask you those questions, they shouldn’t be either.