The notion of a shared residence, or “household” must feature into any sociological understanding of a family. Without this important distinction, there is nothing to separate a family from a non-family social group.
Not all households are families, but all families must have a household. A group of university students sharing accommodation are evidently not a family, even if they all chip in to pay bills. Therefore, a shared residence cannot be the only construct that goes towards creating a family, however, it is essential for the distinction from groups of friends and lovers that might be in a sexual relationship and economically entwined but do not share a residence.
If the three criteria to become a family are shared finances, shared accommodation, and sexual relations, a group of students sharing accommodation could meet all the criteria. If they are engaging in sexual relations with each other, are sharing the payment of bills, and living under one roof, they would meet the criteria for a family group. Does this mean they can be considered a family for sociological purposes? Additionally, when children grow up and leave home, they are no longer be living under a shared roof as their siblings and parents, but few would say they stop being a family. Therefore, a shared residence should not be a critical component of a family.
[P1] Non-familial groups can be economically entwined, and in a sexual relationship, without being a family. [P2] Therefore, the additional criterion of a shared residence is essential for differentiating between non-familial groups and family units.
Rejecting the premises
[Rejecting P2] This does not solve the issues. It is possible to be a non-family group with shared finances, shared accommodation, and a reproductive relationship. It is also possible to be a family without a shared accommodation.