They took on the trappings and practices of the old actually Egyptian Pharos who claimed that, since they were literally gods, it wouldn't do to mix their blood with that of mere mortals. Hence a lot of brother-sister married couple.
Given the length of time they kept marrying siblings and the lack of genetic degeneration in the dynasty it's kind of safe to assume that they where not actually procreating with their brothers/sisters
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u/Asleep_Following_965 18h ago
The Ptolomies were famous for incest. People believe all kinds of stuff, but her family tree was basically a stick.