Reasons for sibling marriage during the Hecatomnids dynasty?

Jun 2019
Good evening,

for quite some time I've read everything that I could get my hands on about the dynasty of the Hecatomnids that ruled Caria (Anatolia) between 395 and 335 BC.

What I never understood and never could find an explanation for is the fact, that all of Hecatomnus' children married each other; except for the last son, who was the only one who had a child.

The question is: WHY???
My findings are the following:

1) Both Hornblower and King write, that there are NO strong matriarchic structures; men were the satraps; a marriage because of heritage or claim to power seem unlikely.

2) Furthermore sibling-marriage was uncommon during this period and in this area. They were even frowned upon - understandibly. So why would a ruler, somebody who wanted his pupil to pay him respect do something as horrible as marry their sister?

3) Even if Mausolus and Artemisia II did not grow up together (I somewhere read that Mausolus grew up at a different king's court) I find it highly unlikely that they fell in love and married... and that the same happened to Idrieus and Ada...

4) No children were born to the sibling-pairs Mausolus and Artemisia II and Idrieus and Ada.
So the marriages were obviously not made to produce pure-blood offspring (like in ancient egypt).

5) There is no evidence for polygamy or chrildren from different wifes.

6) I don't understand how ANY leader/king/satrap could willingly stay childless and give their throne to their brother. (Again: WHY would anybody do this?)

In my opinion it would have made much more sense to marry whom ever they wanted, produce offspring and put their offspring (Mausolus' son eg) on the throne.

Marrying to die childless and let your brother - who dies childless too - reign seems absurd.

Does anybody know an explanation?

kind regards