Limiting families to only those related by blood, those living in a shared accommodation, or those which raise children, are exclusive definitions of family which fail to accommodate all the nuanced family setups of the modern world.
But allowing any unit that performs the core functions of a family unit to be classified as a family, the functional definition can be more inclusive. It means foster parents, that may not be legally or biologically related (or may not even be living under the same roof any longer) can still be classified as a family. It means people who did not form bonds with their biological parents or grandparents, but found a new “family” that provided support and shared resources have also entered a new family, as it provides all the functions of a family. It also means that whoever provides care for frail or elderly “family” members, whether they are biologically related, lifelong friends, or close companions, can meet the functional definition for a family.
[P1] Modern families take many forms. [P2] Rather than focus on definitions that define what a family 'is', sociologists should focus on what they 'do'. [P3] This is the only way to include the many forms of a family unit in the sociological definition of a family.