(adj.) fictional, imaginary; non-blood related
Everyone has family friends they know or knew as aunts and uncles, but who are not actually related to them blood. These are your fictive kin.
That’s a term from anthropology, where the role of fictive kin in rearing children is a frequent area of study. Outside of this fairly specialized used of the word, however, fictive itself is essentially just a synonym for fictional; both words stem from fictio, a rather general Latin word for something formed or devised from raw materials.
When it first appeared in our language in the fifteenth century, fictive simply meant ‘feigned’, ‘imaginary’, or in its earliest sense, ‘given to feigning, or engaging in pretence’. So your fictive kin are essentially ‘feigned’ relatives—people who take on the role of a blood relative, despite not being genetically related to your family group at all. Despite that relative distance from a family’s gene pool, of course, fictive relatives are still hugely significant family figures, and their role in children’s development is often no less important—nor any less appreciated—than ‘true’ aunts and uncles.