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Sibling is a generic term meaning brother or sister, i.e. someone who shares a father and a mother with the person being referenced. Generally, this means that the two people are very closely genetically related, though, in the case of adoption, this isn't necessarily the case.
In most world societies, siblings usually grow up in the same household. This closeness usually means they develop intense feelings about one another - whether love or enmity. However, closeness may not always develop in sibling relationships, particularly between siblings with an age difference of five years or more.
A step-sibling (step-brother or step-sister), is. a sibling to whom you bear no blood relation, but one of whose parents has remarried to one of yours.
A half-sibling (half-brother or half-sister) is a sibling with one shared parent. Half-siblings can have a wide variety of relationships, from a bond as close as any full siblings, to near or total strangers.
While many half-siblings are step-siblings and vice versa, someone may have either relationship without the other: step-siblings each have a parent married to a parent of the other but not necessarily a parent in common, and half-siblings not living in the same household are not always considered step-siblings.