OMG we just found something incredible! We discovered this book and just watch chapter 5: Affinities and Extremes. Crisscrossing the Bittersweet Ethnology of East Indies History, Hindu-Balinese Culture, and Indo-European Allure (by James A. Boon).

From the information we could find, this book shows that marriage between fraternal twins of different sexes was authorized and even encouraged in traditional Balinese culture. This is the first example we heard of an incestuous relationship to be both considered normal and not reserved for the nobility. That's incredible!

Unfortunately, we haven't been able to find an online version (and we're not rich, so between the price of the book, which is already expensive, plus shipping costs to France...), so we don't know much more. But if anyone has this book, we'd be very interested to know what you thought of it!