YOU SAID:

My mother's sister is my favorite aunt, but my non existent sister is her favorite niece.

INTO JAPANESE

私の母の妹が僕の好きなおばさんが、存在しない妹が好きな姪。

BACK INTO ENGLISH

My favorite aunt does not exist is the sister of my mother's sister's favorite niece.

INTO JAPANESE

私の好きな叔母が存在しない私の母の妹のお気に入りの姪の妹です。

BACK INTO ENGLISH

I do not have my favorite aunt mother's sister's favorite niece's sisters are.

INTO JAPANESE

私は私のお気に入りを持っていない叔母母の姉の好きな姪の姉妹します。

BACK INTO ENGLISH

I do not have my favorite aunt mother's sister's favorite niece and sisters.

INTO JAPANESE

私の好きな叔母は母の妹の好きな姪と姉妹とはありません。

BACK INTO ENGLISH

My favorite aunt mother's sister's favorite niece and sister.

INTO JAPANESE

私の好きな叔母は母の妹のお気に入りの姪と妹。

BACK INTO ENGLISH

My favorite aunt's mother's sister's favorite niece and sister.

INTO JAPANESE

私の好きな叔母の母の姉の好きな姪と妹。

BACK INTO ENGLISH

My favorite aunt's mother's sister's favorite niece and sister.