FL x demon lord

Although the title doesn't explain much, this story is about an MC reincarnated into a book world where she will become the saintess. She promptly dumps the boyfriend (a soldier bit also the male lead in a harem-style story) - and then unbeknownst to her ends up with the demon lord.

# Saintess x demon lord

FL gets reincarnated into a book as the unfortunate blood-related sister to one of the story's main characters. Her role is to have a miserable and short life and become a source of drive for the brother. But this FL doesn't want to die, so she sets about changing the fate of her character and in the process draws the attention of 3 hot guys.

# 3 potential love interests where one of them is the demon lord.

Human females are forced to travel to the demon world and become wives of demons as part of a peace treaty. Story centers around FL and her two friends as well as their partners. FL's partner is the demon lord.
Interesting story.

Technically the ML here is not the demon lord, but everyone else calls him so. FL gets married to him and together they play along with the demon lord rumors and use it to their advantage.