Incest between brother & sister was especially Egyptian royalty. Incest of various kinds though, was common in other cultures as well. I believe it was the Jews who first made laws against it.
The Canaanite peoples the Hebrews displaced actually worshiped brother & sister gods which were also husband and wife....
I knew about the Canaanites, didn’t think it was as common as you indicate.
Learn something new every day!