YOU SAID:

someone has to stop my boyfriend from sleeping with my best friends sister

INTO JAPANESE

私のボーイフレンドが私の親友の妹と寝るのを誰かが止めなければなりません

BACK INTO ENGLISH

Someone has to stop my boyfriend from sleeping with my best friend's sister

INTO JAPANESE

私のボーイフレンドが親友の妹と寝るのを誰かが止めなければなりません

BACK INTO ENGLISH

Someone has to stop my boyfriend from sleeping with his best friend's sister

INTO JAPANESE

私の彼氏が親友の妹と寝るのを誰かが止めなければなりません

BACK INTO ENGLISH

Someone has to stop my boyfriend from sleeping with his best friend's sister