YOU SAID:

Heavy treat that child like his own younger sister.

INTO JAPANESE

その子供を自分の妹のように重く扱います。

BACK INTO ENGLISH

Treat the child as heavy as his sister.

INTO JAPANESE

子供を妹と同じくらい重い扱いをしなさい。

BACK INTO ENGLISH

Treat your child as heavy as your sister.

INTO JAPANESE

あなたの子供をあなたの妹と同じくらい重いものとして扱ってください。

BACK INTO ENGLISH

Treat your child as heavy as your sister.