Flowers of tragedy

When the snow cleared, Kanae noticed that Shinobu was no longer trying to attack her, instead she was trying to crawl away. The young girl latched onto his waist and faced their attacker. She was shocked to see that the assailant was a young woman not much older than herself wearing a haori that was pink with flowers. The young woman's blue-green eyes glared daggers into the siblings.
"Are you alright? I could have impaled you." Her voice was light, but stern.
Kanae yelled back. "Why did you attack us?"
"Because that girl is now a demon, a creature of darkness who craves human flesh. Demons are born when their blood is mixed with the blood of another demon." The woman pointed her sword at Shinobu's body. "And it is my job as a Demon Slayer to eliminate all such monstrosities."
When young Kanae Kochou returns from a trip into town to find her father dead and her younger sister a demon, she swears revenge against the monster who committed the crime and sets off on a journey like no one has ever seen.
(Aka, a KNY role swap AU that swaps every character and even some locations, not just the main characters)