Not exactly an idea for a new bad ending, but a modification in an existing one: Noriko - Revenge path: After learning of Noriko's deal, you decide to pay her back with her own medicine, you make a deal with your tormentors and help them get their hands on her.
One reason i suggest this version is it would implicate the protagonist more in her fate as well as drawing parallels between the siblings. It would also put the protagonist under the thugs' influence, basically the same position Noriko was in.
Another reason is that i think we should be able to be cold towards her and still save her not because we like her now, but because she is family, despite us still not liking her.