Are you joking?

I'm thinking of the first time Qiu Yun met us.

You introduced yourself. Then you introduced me.
You: This is Shen. She's my girlfriend.
Qiu Yun: Are you joking?

Jacob: Why you ask this kind of question?
Qiu Yun: Because sometimes people joke what. Could be his sister what. Are you his sister?
You: I really am not joking. She really is my girlfriend.