I thought the point of a sin was that you are doing something forbidden..
Forbidden is stuff you know you do. Sin is a religious concept that's really nothing more than a guilt trip.
a sin hurts people and i guess something forbidden might hurt only you in the long run... that is the difference i would put there...
sometimes something forbidden is a sin and sometimes it's not, however a sin should always be forbidden...unless it's just really good chocolate cake ;)