Why does Hebrew use the same letter to represent two different sounds: shin and sin? As far as I know, with the exception of when using a dagesh kal, no other Hebrew letter has this property.
http://ancient-hebrew.org/36_creation.html , which I realize a lot of people would not consider authoritive has this idea:
The tongue closes a circle for the Samech, while air streams out from both sides of the upraised tongue for the Shin and Sin. Thus the graphic resembling two air streams split by the upraised tongue in the middle of the mouth.
So same form for the two sounds, just change the dot position to show if you should be breathing in or out for the sound.