Trying to work out existing light switch wiring with possible loop
Not a direct answer, but a way to clue you in if nobody else comes
  along with an answer:


I had to diagnose the wiring on my house when I first moved in. What I did, I first turned off the power on the circuit, then after being sure it was off, I connected a little 9V battery across two wires. 

I had my wife stand at the other light switch, and with a multimeter (or voltmeter) check the wires to find which two I was holding in my hands. If you find +9V, the wire contacting the positive lead of the multimeter is the wire touching the positive side of the battery. If you find -9V, the wire contacting the positive lead of the multimeter is the wire touching the negative side of the battery. 

If you identify all your wires, and what goes to where, you can make sense of it by drawing a diagram.