I'm Napoleon Among Demon Kings

The Supreme Demon King, the Bearer of Fire, the Key's Son, the One Who Turned into a Thousand Bones, and the protagonist of the biography He Changed the Demon Race: Ye Cheng Sotos is assassinated by a human assassin one night!

Then he becomes the Silver-White Sword Princess's fiance and older brother?

Also known as Remember, Don't Change Your Household Registration Recklessly; Check the Family Tree Before You Forge an Identity; This Is Even More Terrifying Than Marrying a Minotaur; I Want to Go Back to the Demon Race