iphone 4 passcode reset

i purchased an iphone 4 from a friend of mine for my son to use as a play phone, my sister asked to use the phone until hers got fixed. she logged into her icloud and her crazy guardian found out she had a phone and hacked the account and locked the iphone with a passcode and i guess put it in lost mode. i tried resetting the iphone on itunes but my sister doesnt remember the email to her icloud from 3 years ago. any suggestions on how i can get the passcode off or reset? like i said it is an old iphone 4 that my son was using for a play phone. thanks.