Sociologists would describe them as "fictive kin".
Fictive kinship refers to a term which is utilized by to portray types of family relationship or social ties that depend on neither consanguineal (blood ties) nor affinal ("by marriage") ties, as opposed to genuine family relationship ties.The social universe built up by family relationship can't be characterized exclusively as far as science and marriage alone. Without a doubt, connection sets up the base, however not the totality, of what people consider as family. The parts that family plays in a general public are not finished without the incorporation of fictive kin relationships.