YOU SAID:

my sister think im stupid

INTO JAPANESE

私の妹は愚かな考え

BACK INTO ENGLISH

My sister thought stupid

INTO JAPANESE

私の妹は愚かだと思った

BACK INTO ENGLISH

I thought my sister was stupid.

INTO JAPANESE

私は妹が愚かだと思った。

BACK INTO ENGLISH

I thought my sister was stupid.