The most evident proof of the importance in Ancient Egypt of Isis and Nephthys in a rebirth process is in the Books of the Day and Night, which describe the journey of the sun god through the sky.
According to the thought of Ancient Egypt, especially during the New Kingdom, Nut was the goddess of the sky, so the sun made a journey through the goddess’ body.
The dusk happened because Nut swallowed the solar disk and during the night he traveled all over the Nut’s belly. The morning after, the sunrise meant that Nut was giving birth the solar disk. That is the iconography that the artist of Ancient Egypt depicted on the ceilings of tombs from XX Dynasty.
But Nut was also Osiris’ mother and the resurrection of the dead in Ancient Egypt happened because the corpse was assimilated to Osiris, so the new-born was Osiris, son of Nut, who was assisted by Isis and Nephthys.
Taking that into consideration, it make sense that the priests of XX Dynasty included the figures of Isis and Nephthys in the sun disk rebirth. As a consequence the artists of Ancient Egypt had to create a new iconography with the union of the sun rebirth and the Osirian tradition of the two divine mourners.