Cleopatra (VII), the last queen of Egypt, was forced to marry her younger brother to keep the royal blood in the family. This, of course was a form of royal incest.
From what I understand, the practice of royal incest barely existed in Egypt before the arrival of the Ptolemaics. These represented a Greek royal line descended from Ptolemy, a general of Alexander the Great. And the Hellenic and Egyptian cultures were quite different. So how/what caused the Greek Ptolemaics to revive the barely existent Egyptian practice of royal incest?
And even if, as a commenter noted below, the Ptolemaics chose to adopt a "local" custom to consolidate their power, why this particular one?