It's funny that you just posted this because as I was reading the paper this morning, I came across "which" used as a relative pronoun modifying "citizens" and I wondered to myself if it was possible!
As a teacher, my first response is 'no' because obviously, a family is comprised of people, not things (just as "citizens" are people), so "who" or "that" is correct.
I do wonder, however, if the "family" as unit can be considered a thing...
Any thoughts out there?
Student or Learner