To define the nucleus of a family, as we all know the traditional nuclear family has changed quite a bit over a while. It really is those immediate people who provide them the support, and they're doing it 24/7. They're the ones they turn to first. When they're sick, they're the ones who go to the drugstore or drive them to their appointments.
To me, it doesn't necessarily have to be the tradition of the spouse and the children. It could be parents, it could be common-law; whatever the orientation is fine. But it's those immediate people who are providing the support who should be recognized as that family unit if they're sharing a similar dwelling or they're in their lives every day.