It is important to know that there is no such thing as an Egyptian mythology. Or rather, there is not ONE Egyptian Mythology. In the history of the Egyptian Empire, several cities played different roles at different times. There were Thebe, Memphis, Heliopolis, Hermopolis, and many others... Each of these cities had its own stories, and its own deities. The principal deities would occur in the stories of all of them, but their relationship and the part they played, could be entirely different in each city.
In some stories, e.g., Ra is the son of Ptah, and the father of Seth, Nephtys, Isis and Osiris. Other stories make him the son of Nun and Atum. Ra, himself, then, got two children, Shu and Tefnut, who became the parents of Nut and Geb. These two, in turn, became the parents of Seth, Nephtys, Isis and Osiris, making those four Ra's after-grandchildren, instead of his children...
Or, to give another example: Nuit has been called Ra's wife, his daughter, and his granddaughter. The same goes for Hathor/Sekhmet. In addition to that, some myths claim that Nut was Hathor's mother, other say she was her sister, and still others "leave no doubt" that the two were half-sisters, one of them being created, and the other conceived by Ra...
So, more than this is the case with other mythologies, it is, in Egyptian mythology, difficult to say who each deity is, and what quality and/or energy he or she represents. The position, lineage and meaning of each deity varied, depending on the city, and the moment in time.
CREATION STORIES: there is not one creation story. The stories were different in each town. Two examples.
a) First there was Nun, the timeless primordial ocean. Then came Atum, the first mountain. Together they had some children, of which Toth and Ra are the most famous. Ra then created Shu and Tefnut, who were to become the parents of Nut and Geb. Nut and Geb got four, or according to some five, children, Seth, Nephtys, Isis and Osiris. Hathor/Sekhmet would then have been the fifth child, a firstborn daughter who later on became Ra's wife.
b) Other cities had a simplified version of this. Before time, there was Ptah. He was the father of creation, and the father of Toth and Ra. Ra, then, created Nut (heaven) and Geb (Earth), and became the father of the Fab Four. According to some they were the children of Ra and Hathor, who was Ra's first daughter, or creation.
Credits: a lot of the material on Egyptian Mythology comes from the "Egyptian Mythology FAQ," by Shawn C. Knight.