Snapping Point (A Yandere Big Brother)

I have been upsest with yandere for all my life and I just couldn't help myself. Seiko is a 17 year old highschooler who has a big brother named Toma (Name based off of Amnesia character) that is 18. Everything was fine for a few years but Seiko began to see why he was different around her. "I love you my sweet baby...