Satsuki detailed to me everything that happened that day. Her sister's boyfriend was already like a regular visitor to their house but when they broke up, the guy kept on pursuing her sister. Stalking her, even.
One day, when their parents left for a trip and Sakuma went out to see the house his parents picked, only Satsuki and her sister were the ones left at the house. They still shared the same room at that time, being a high-schooler and grade-schooler, though they're close, the age gap put a considerable distance between the sisters. Satsuki also told how her sister had a complex against her. Unlike her, her sister didn't have any growth spurt so, at that time, they already have the same body build.
And that's when it happened. Her sister's ex who was stalking her for days took that