That, my dear fellow, is an author and mangaka diving
deep into folklore and myth, making full use of the "seinen"tag... 😋
If you read your ancient creation myths, you'll find plenty cases where exactly this happens.. Egyptian, Assyrian/Mesopotamian, various flavours in Asia, and gods only know what the Hindus got up to.
All center around a single central theme.. Brother gets killed because [x], sister preserves head through magic, proceeds to resurrect the body,
always including
sex errmm.. "Love", then "reborn" brother ascends to Godhood and rules forever.
No doubt the author can point
exactly at the specific instance of this in the wider chino-japanese mythology and folklore, citing chapter and verse to flummox accusions of Perversion and all that.
It's
most certainly a thing the shinogami were accused of/employed for. So there's actual history behind it as well.