Ten years ago.
"Of course, I like you."
"Oh, I like you too."
"Really?"
"Nonsense! You are my brother. If I don't like you, who will like you!"
Ten years later.
"What, Brother Yang, do you like me?"
"Yeah."
"Really?"
"You are my sister."
Tsk, I lied to you, so did I.
Concept: I love you, so I can't love you.