YOU SAID:

I never laugh, 'cause to laugh is the sister-in-law of counterpart.

INTO JAPANESE

私は絶対に笑わない、だって笑うのは相手の義姉だから。

BACK INTO ENGLISH

I never laugh because it's the other person's sister-in-law who laughs.

INTO JAPANESE

笑うのは相手の義姉だから絶対に笑わない。

BACK INTO ENGLISH

I never laugh because it's the other person's sister-in-law who laughs.