YOU SAID:

My sister thinks i am stupid

INTO JAPANESE

私の妹は私がばかだと思う

BACK INTO ENGLISH

My sister thinks I'm an idiot

INTO JAPANESE

私の妹は私がばかだと思います

BACK INTO ENGLISH

I think my sister is an idiot

INTO JAPANESE

私の妹はばかだと思う

BACK INTO ENGLISH

I think my sister is an idiot