YOU SAID:
I am having an affair with my niece.
INTO JAPANESE
私は姪と関係を持っています。
BACK INTO ENGLISH
I have a relationship with my niece.
INTO JAPANESE
私は姪と関係があります。
BACK INTO ENGLISH
I have something to do with my niece.
INTO JAPANESE
私は姪と関係があります。
BACK INTO ENGLISH
I have something to do with my niece.
That didn't even make that much sense in English.