what family is

Family is a group consisting of two parents and their children living together as a unit. or
Family is a group of people related by blood or marriage.

This is the complete definition of 'family' from oxford dictionary:

Definition of family

noun (plural families)

• 1 [treated as singular or plural] a group consisting of two parents and their children living together as a unit:she moved in with her boyfriend’s family[as modifier]:the family home
• a group of people related by blood or marriage:friends and family can provide supportI could not turn him away, for he was family
• the children of a person or couple being discussed:she has the sole responsibility for a large family
• informal a local organizational unit of the Mafia or other large criminal group.

• 2all the descendants of a common ancestor:the house has been owned by the same family for 300 years
• a group of peoples from a common stock.

• 3a group of related things:all manuscripts that share this reading constitute a family
• Biology a principal taxonomic category that ranks above genus and below order, usually ending in -idae (in zoology) or -aceae (in botany):the cabbage family
• all the languages ultimately derived from a particular early language, regarded as a group:the Austronesian language family
• Mathematics a group of curves or surfaces obtained by varying the value of a constant in the equation generating them.

adjective

[attributive]

• designed to be suitable for children as well as adults:a family newspaper

Phrases

the (or one's) family jewels

informal a man’s genitals.

in the family way

informal pregnant:he would disgrace them by getting a girl in the family way

sell (or sell off) the family silver

part with a valuable resource for immediate advantage:the suggestion that privatization is selling off the family silver