YOU SAID:
i've heard he's been sleeping with your sister
INTO JAPANESE
彼はあなたの妹と寝ていると聞いた
BACK INTO ENGLISH
I heard he slept with your sister
INTO JAPANESE
彼はあなたの妹と寝たと聞きました
BACK INTO ENGLISH
I heard he slept with your sister
That didn't even make that much sense in English.