Not sure why so many people are upset with a character death in a visual
novel.
More significant characters than the sister die all the time, whether in great stories; Achilles/Hector (the Iliad), Hamlet, Macbeth, Othello, Juliet (Shakespeare), Lenny (Of Mice and Men), Beth (Little Women) or contemporary ones; Dumbledore (Potter - verse), Uncle Ben/Gwen Stacy (Spider-Man) etc.
If it is a part of the story the author wishes to tell so be it IMO; and, if as I suspect it will lead to a crusade against whatever she was investigating later in the game, it'll actually serve more purpose than the death in that game by, as
Canto Forte likes to call him, Pink Baker.