What is the sociological definition of a family?

A common residence differentiates family from close friends

Question: What is the sociological definition of a family?
Position‹2 of 6›: Family means a shared residence, economic cooperation, and reproduction
Argument1 of 4›: A common residence differentiates family from close friends

A shared residence is essential for the distinction of family from non-familial friendship groups.

The argument

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.

Context

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.

Premises

[P1]Not all households are families, but every family must have a household. [P2] University flatmates who share bills are not a family, so a shared residence alone cannot constitute one. [P3] Yet a common residence is still essential to distinguish a family from friends or lovers who are economically and sexually entwined but live apart. [C] Therefore, a common residence is part of what defines a family — a shared residence, economic cooperation, and reproduction.

Counter-arguments

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.

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.