[Pearls Before Swine]

In Islamic countries you and your brother could marry each other’s daughters.

I think that makes normal genealogical relations very confusing. The business analogy would be Japanese keiretsu, or Korean chaebol. It's fun at first, but there are downsides.

[Verginius Rufus]

The Emperor Claudius' fourth wife was his own niece--a lot of Romans seem to have considered that incestuous. (She was the mother of the future emperor Nero by a previous marriage.)

Spartan kings seem to have married their nieces sometimes--at least Leonidas, famous for his heroic death at Thermopylae in 480 B.C., married his brother's daughter.

2 Samuel 13.13 implies that Amnon could have married his half-sister Tamar, but that is contradicted by Lev. 18.11.

[Verginius Rufus]

The Emperor Claudius' fourth wife was his own niece--a lot of Romans seem to have considered that incestuous. (She was the mother of the future emperor Nero by a previous marriage.)

Spartan kings seem to have married their nieces sometimes--at least Leonidas, famous for his heroic death at Thermopylae in 480 B.C., married his brother's daughter.

2 Samuel 13.13 implies that Amnon could have married his half-sister Tamar, but that is contradicted by Lev. 18.11.