Sister ni Natta Onna Kishi no Hanashi //// - Ch. 1

[MangaDex]

 

[FloorRoof]

This isn't exactly safe for work... The knight's name even hints where this is going.
And it's slightly odd that the hero was the first victim. For a moment I thought the monster having that kind of a preference would be the joke.

 

[kirby_422]

I expected the other kind of sister instead of nun.

 

[hybras]

I expected her role change went to mother of the hero's bloodline.

 

[Kaarme]

That's certainly a straightforward name for the former knight.