13
Short Answer
There is no clear primary source evidence for either why the Hecatomnid dynasty (c. 395 - 334 BC) siblings married each other or why it was 'permitted' (by which I take it to mean why the citizens didn't drum the siblings out of town), but the former question is discussed in some detail by E. D. Carney in Women and Dunasteia in Caria. While ...
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