Jolyne noticed that her sister, Karuki was acting weird so recently, when she checked on her, she noticed that Karuki was staying silent in her room and didn't talk that much as usual, then Jolyne found out that Karuki was stress of herself that she wasn't able to do things right, Jolyne cheered her up and promised her that she would always be there to support her, whatever she needed her. So, Jolyne was willing to kill anybody who attempts to hurt her precious little sister...
Edit: Karuki was trying to ignored her trauma but she wasn't able to handle it and got stressed, which causes Jolyne to go Yandere Mode
BONUS: This fanart isn't mine, I kinda borrowed it with my Online Bestie