YOU SAID:

brother, you better be joking

INTO JAPANESE

兄、冗談でしょ

BACK INTO ENGLISH

Brother, you're kidding.

INTO JAPANESE

お兄ちゃん、冗談でしょ。

BACK INTO ENGLISH

Onii-chan, you're kidding.

INTO JAPANESE

お兄ちゃん、冗談ですよ。

BACK INTO ENGLISH

Onii-chan, you're kidding.