I loved it too. I do think Yuki may have pushed her. I certainly would have. She was a selfish, destructive woman who enjoyed destroying lives as a form of entertainment. She didn't really love Yuki as much as she loved the power and control she held over him. The same could be said for Karou. She was clearly a psychopath but wealth kept her from treatment and punishment. Well, until she pushed Yuki too far. Yuki and Karou need each other's forgiveness, but they need to forgive themselves just as much. The sister caused her own death, ultimately. One can't play with another person's life so cruelly and not expect some form of retribution. At least, that is my opinion and experience. Kick a dog a little and it will submit, but kick it too much and it will bite your leg off. Kinda makes you wonder what her parents were like, ya know? Was it their desire to spoil her the catalyst of her cruelty or did they give into her every demand because they feared her cruelty?
I loved this story and imo Karou wasn't really twisted but more desperate in the fact that he would take any measure to be with Yuki ie marrying his sister. Anyway, the art was beautiful but one question I have is did Yuki push his sister?