Just wondering since the Pharaoh was a "God King" and usurping that title would be rather difficult for someone born outside the Royal Family to suddenly say they were chosen by the Gods, although I know one of the Tuthmosis by saying the Sphnix of Giza chose him as Pharaoh. Was it similar to the Chinese Mandate of Heaven or was the whole idea of a "God King" not strictly adhered to by the Egyptian Nobility and/or Peasantry?
To a degree, succession was a matter of marriage. While the men ruled the nation, it was often marrying the right woman that made you king of Egypt. For example, after the death of Tutankhamun, it was Ay marrying Ankhesenamun (Tut's sister/wife) that made him the new king. It typically requires a major social upheaval to see someone completely unrelated on the throne.
To illustrate that point, I'll go back to the Eighteenth Dynasty. After Ay's reign, he intended to be succeeded by Nakhtmin, who was likely an adopted son. However, the Eighteenth Dynasty as a whole was not terribly popular in Egypt as a result of the events at Amarna, so the adopted son of a guy who became king only because he married the wife of the previous king wasn't a very compelling case, so no one was terribly upset when Horemheb, the commander of the army, seized power instead.
Also, it should be pointed out that Tuthmosis IV was not from outside the family. He merely assumed the throne, possibly by overthrowing his older brother, and then used the Sphinx story as justification. The Eighteenth Dynasty was weird. Great to read about, but weird.