YOU SAID:
your sister is more stupider
INTO JAPANESE
あなたの妹はより stupider です。
BACK INTO ENGLISH
Is your sister is stupider.
INTO JAPANESE
あなたの妹が stupider です。
BACK INTO ENGLISH
Your sister is stupider.
INTO JAPANESE
あなたの妹は stupider です。
BACK INTO ENGLISH
Your sister is stupider.
That didn't even make that much sense in English.