Fictive kin is a
family networking/
social science term for people who are not related by
blood or
law, but by
social means, who function the way other traditional
family members would. They are non-relations who reach
family member status.
Good examples of these people are godparents or godchildren. However most fictive kin have even less formal connections than that of a godparent. These are the people who your parents told you to call 'uncle' or 'aunt', who are just good family friends.
Some families from different ethnic backgrounds tend to have more fictive kin in the family. In the United States, black families tend to have more fictive members. However like most social service assumptions, your mileage may vary.