Yandere Little Brother x Older Sister FemReader

10 years ago, your parents died from a terrible car wreck and now it's your job to take care of your little brother, Nikku. For so long you always say that he's a sweet, kind, and wonderful little brother he is. But that's what you think. Since he starts being around with you for way too long, he really doesn't want you to leave because he's very obsessed with you. Then suddenly, he became too attached to you like glue. What made him like this?