I think it was her naivety and her young age that made her that way. She loved a man who was forced to be engaged to her yet he had feelings for her sister. It was obvious that she wasn't a good match for him but she deluded herself because she "loved" him. She never got to mature or grow up and her sickness made her feel worse, that she was losing everything. All she felt she could do was feel bad for herself and make them feel guilty for living. Pretty sad she never truly talked to anyone about her feelings, maybe she would've had a better end of her life, putting her feelings out there.
Agreed, though the little sister's "love" was actually just obsession. Otherwise she would have actually cared about his happiness and not lied to herself about it. And she wouldn't have hated her sister for how the fiancé shows different reactions when he is with the older one compared to how he behaves with her. His interest towards the older sister isn't even visible enough to result in such intense jealousy and hatred as it did but as you wrote before, her being sick and making her feel like she could loose everything at any time does explain how the feeling of jealousy could rise to such under other circumstances inexplicable hights. Still proves what kind of character the younger sister actually has and it's not a good one. Sickness truly shows a person's true colours...
I do understand where the little sister's coming from but I don't like her one bit! Lmao the fact that her husband was falling for her big sis and she hated her for that....