AA.VV 
The Burden of Isis [EPUB ebook] 
Being the laments of Isis and Nephthys

Of all the Egyptian religious beliefs that existed from the Prehistoric period down into Roman times, the oldest and the one most held in veneration was that connected with the worship of Osiris, Isis and Horus. These three, though primarily only local gods, at an. early period became prominent deities of all Egypt; and the cult of Isis, more particularly, remained a favourite always, rivalling even that of Osiris in later times.
During the many thousand years of Egyptian history, not only did many changes occur in the ceremonies connected with these cults, but also the legends and origin of the Osirian faith received many additions and interpolations; and thus the old faith lost much of its purity. The simplest form in which it is preserved to us states that Osiris was the son of Seb and Nut–i.e. Earth and Heaven; of whom were
born also Isis, Nephthys and Set, or Sutekh, as he is also called, Osiris married his sister Isis, while Set chose Nephthys. It is probably due to this feature of the legend that the Pharaohs often married their sisters, and occasionally also their daughters. Osiris was the first divine ruler of Egypt.