We have adopted siblings, foster siblings, even something called fictive siblings, like a blood brother. Fictive siblings are accepted into a family through custom or just plain choice. So being a sibling is not just blood but some degree of common genes, common history, common family values and culture or a legal status.
Most research has been done with full siblings ignoring half siblings, stepsiblings, adoptive siblings or fictive siblings. These fictive siblings are siblings that have been adopted into the family as siblings based on desirability and customs rather than on the basis of blood ties over legal criteria. Yet siblings in most cultures have always had fictive siblings.
Think of the concept of blood brothers, sorority sisters or nuns called " sisters" in the Catholic church In some cultures these fictive siblings are sentimental or honorary yet have all the privileges of full siblings. Gay and Lesbian families at times create sibling ties with friends.
A better way to look at siblings and indeed families is from the point of view of kinship or the anthropological lens. With the breakdown of the nuclear family in our culture and the growth of single families, the concept of extended families is a better prism .
Kinship seems to fit the family of the 21st century best. This takes in the fact that fictive siblings from the neighborhood, or cousins in minority families are often looked at as siblings and are also defined as family.