I am locked out of my iphone 5 and find my iphone is disabled because I do not know the apple ID associated with the phone.

My brother set a passcode on his iPhone but forgot what it was. Now we are locked out and the screen is telling us to connect to iTunes. We have never plugged this phone into a computer to synch it with iTunes and the sales associate at the store turned off the find my iPhone feature when the phone was transferred from my sister's number to my brother's. We are unsure of the apple ID that is associated with the phone and would love to know how we can go about unlocking it/resetting it.