I guess that's to be expected of someone used to that level of kindness all her life? She takes it for granted. I think she knew that no matter what she did Natsumi will always love and dote on her. But tougo, I feel she always knew, was never interested in her and was just doing the bare minimum and she actually had to work hard to deserve the attention she got from him. And it was frustrating to see her able bodied sister easily get that attention by being herself. So it makes sense that she pits herself against her sister because it's always been them against the world but they're so vastly different.
Maybe, she feels that if she hadn't been sick she could have grown up into a mature adult too and the frustration and fear that she never can causes her to feel like that. So it's almost common sense that she wants her sister to die and not her fiance.
The part when haru said she wanted to drag her sister down to hell with her gets me so heated. LIKE BRUH SHE LOVED AND CARED FOR YOU ALL YOUR LIFE, PRIORITIZED YOU, SUPPORTED AND EVEN DECIDED HER CAREER FOR YOU, and you’re just gonna wish she died too so she wouldn’t fall in love with your fiancée who doesn’t even love you??? WHAT THE FUCK BRUH