YOU SAID:

I wonder what level of Mario Party my cousin is on

INTO JAPANESE

私のいとこはマリオパーティのどのレベルにいるのかしら

BACK INTO ENGLISH

I wonder what level my cousin is in mario party

INTO JAPANESE

いとこはマリオパーティでどのレベルなのかな

BACK INTO ENGLISH

What level is your cousin in Mario Party?

INTO JAPANESE

あなたの従兄弟はマリオパーティで何レベルですか?

BACK INTO ENGLISH

What level is your cousin in Mario Party?