YOU SAID:
My brother is the worst person on this world
INTO JAPANESE
私の兄はこの世界の最悪な人
BACK INTO ENGLISH
My brother is the worst people in this world.
INTO JAPANESE
私の兄は、この世界で最悪の人々 です。
BACK INTO ENGLISH
My brother is the worst people in this world.
That didn't even make that much sense in English.