Zhang Yuan actually didn't like Zhang Yuhuan when he was a child. After all, as a boy, how could he like someone who thinks of himself as a girl?
When Zhang Yuhuan grows up, he doesn't want to call Zhang Yuan sister at all, because...
"Sister, didn't you say you wanted to sleep together before? Come on!"
"No! You are not my sister! My sister would not want to sleep with me!"