Julius Caesar: Britons had wives in common
“Ten and even twelve have wives common to them, and particularly brothers among brothers, and parents among their children.”
The most civilized of all these nations are they who inhabit Kent, which is entirely a maritime district, nor do they differ much from the Gallic customs. Most of the inland inhabitants do not sow corn, but live on milk and flesh, and are clad with skins. All the Britons, indeed, dye themselves with woad, which occasions a bluish color, and thereby have a more terrible appearance in fight. They wear their hair long, and have every part of their body shaved except their head and upper lip. Ten and even twelve have wives common to them, and particularly brothers among brothers, and parents among their children; but if there be any issue by these wives, they are reputed to be the children of those by whom respectively each was first espoused when a virgin.
https://en.wikisource.org/wiki/Commentaries_on_the_Gallic_War/Book_5#14