[MrTerrorist]

Spoiler for c40:

In her moment of madness Yukariko thought by sacrificing her sister, not only she thought she was saving her sister but was stopping the plague as well. But in the end it did nothing, the villagers who buried Yuuko went mad and died from the plague, her father committed suicide over Yuuko's death and Yukariko realize she killed her sister for nothing and made her suffer.

The fact that Shadow Yuuko didn't kill Yukariko when she found her dead body shows that while she hated her sister for what she did, she still love her.

Now the future chapters will have to deal with getting the sisters to see each other again and make them reconcile.