This. Saku Saku as a whole is, to be honest, a forgettable game, but the sister's route was so well done that I still remember it to this day. If you haven't, do try Angelic Chaos. The little sister's route is also quite good there.
The trigger was her finding out that her aunt and uncle killed her parents, overwhelming her with shock and fear. The constant abuse and being locked in the basement didn't help either.