Friends, roommates, or neighbors who are as close as family and fill the function a proper family should, complete with uncondictional love (and frequent bickering). A boon for those with less than happy real families. Similar to the Japanese Nakama; see Brother From Another Mother for a specific relationship.
"...and this is my sister Megan."
"Dude, aren't you an only child?"
"We're unbiological family."