They could be all relationships. But in a more mundane situation, they are referred to as wife and husband. This concept of devoted love was carried over to common people. Devoted lovers called each other brothers and sisters. In Egyptian love-songs the words 'sister' and 'brother' simply mean 'soul mate' and do not denote a blood relationship.
Many ancient writings refer to wives as sisters and husbands as brothers.
In ancient Egypt, a wife-sister judicial status existed whereby a woman, in addition to becoming a man's wife, was adopted by him as a sister. In such a case, two separate documents were drawn up, one for marriage, and the other for sistership, and the woman concerned, merited higher social status and greater privileges than the ordinary wife. Family Members by Feeding In the Middle East, since ancient times, mothers regularly nurse each others' babies when they get hungry. The nursed children will refer to their feeding nurse as 'mother', the same term that they used for their biological mother.