pseudosibling

plural pseudosiblings

1. One who is not a sibling but has a relationship of that kind.

   […] the girls were older than average when they entered the family and thus did not develop the usual pseudosibling relationship […]

   1982, David P. Barash, Sociobiology and behavior

   1 more example

   With multiple stepparents, stepsiblings, half siblings and pseudosiblings, it’s a whole new type of family, and with it comes the challenge to design a home where everyone feels welcome.

   2007 April 19, Kate Murphy, “Stepfamilies Try to Stretch Their Space”, in New York Times