because most mangas/films/books have plot points that YOU KNOW won't happen, like "oh this character dies during climax but you know they're not really dead/will come back to life/won't die/will always win/happy ending" but here nothing is off limits and anyone can die/get hurt/raped/etc so it's unexpected and surprising and also because hopefully both sisters turn into badass girls (unlikely for the younger one but anything can happen so...)
because most mangas/films/books have plot points that YOU KNOW won't happen, like "oh this character dies during climax but you know they're not really dead/will come back to life/won't die/will always win/happy ending" but here nothing is off limits and anyone can die/get hurt/raped/etc so it's unexpected and surprising
and also because hopefully both sisters turn into badass girls (unlikely for the younger one but anything can happen so...)