I'm looking for historical fiction books with the most tangled family trees.

The Bible. It is a giant incest fest.

Don’t know if you are familiar with John Masters, but he wrote a series of historical novels about a British family serving in India on their Imperial Salad days, the Savage family. Lost track of which characters are which, as far as father/son/uncle/nephew, but it runs the gammit.

One Hundred Years of Solitude. The entire point is how things would come to a circle in the end. I think two relatives come into relationship with each other at one point too.

The Camber of Culdi series by Katherine Kurtz.

The Outlander Series by Diana Gabaldon
Link:https://en.wikipedia.org/wiki/Outlander_(book_series)