YOU SAID:

My sister has the whitest teeth I ever came across

INTO JAPANESE

私の妹は私が今までに出会った最も白い歯

BACK INTO ENGLISH

My sister is the whitest teeth I ever came across.

INTO JAPANESE

私の妹は、私が今までに出会った最も白い歯です。

BACK INTO ENGLISH

My sister is the whitest teeth I ever came across.