Isis , Nephtys, Osiris and set were the four children of Geb (earth) and Nut (sky) who in turns are children of Shu (Air) and tefnut (moisture or water) who are all originated from the self creating god Atum who is often associated with Ra.
All of these Deities form the “The Helioploitan Ennead” which is one of the core principles of ancient Egyptian religion.
Isis is the protective goddess, the wife of Osiris and the mother of Horus and Since each pharaoh was considered the 'living Horus', Isis was very important across the ancient Egyptian history. As she protected the pharaohs of Egypt across the dynasties.
The Book of the Dead describes her as ”She who gives birth to heaven and earth, knows the orphan, knows the widow, seeks justice for the poor, and shelter for the weak”
The Osiris myth
The Osiris myth is the most elaborate and influential story in ancient Egyptian mythology. It concerns the murder of the god Osiris, a primeval king of Egypt, and its consequences. Osiris's murderer, his brother Set, usurps his throne.
Meanwhile, Osiris's wife Isis restores her husband's body, allowing him to posthumously conceive a son with her.
The remainder of the story focuses on Horus, the product of the union of Isis and Osiris, who is at first a vulnerable child protected by his mother and then becomes Set's rival for the throne.
Their often violent conflict ends with Horus's triumph, which restores order to Egypt after Set's unrighteous reign and completes the process of Osiris's resurrection.
The myth, with its complex symbolism, is integral to the Egyptian conceptions of kingship and succession, conflict between order and disorder, and especially death and the afterlife. It also expresses the essential character of each of the four deities at its center, and many elements of their worship in ancient Egyptian religion were derived from the myth.
The story of Osiris is everywhere represented on the walls of this temple, and two of its inner chambers are particularly rich in symbolic imagery.
The island Myth
Since Philae was said to be one of the burying-places of Osiris, it was held in high reverence both by the Egyptians to the north and the Nubians to the south.
It was deemed profane for any but priests to dwell there and was accordingly isolated and named "the Unapproachable".
It was reported too that neither birds flew over it nor fish approached its shores.
These were the traditions of a remote period; since in the time of the Ptolemies of Egypt, Philae was so much resorted to, partly by pilgrims to the tomb of Osiris, partly by persons on secular errands, that the priests petitioned Ptolemy Physcon (170-117 BC) to prohibit public functionaries at least from coming there and living at their expense.