A sibling is a biological brother or sister, a stepbrother or stepsister, or an adopted brother or sister. For there to be a sibling relationship, each person must have been born to at least one of the same parents. Siblings don’t need to be from the same biological parents as long as they both became “children” before the age of 16 in the case of adoption, or before the age of 18 in the case of stepchildren.