M.C.
S.,
I know this might sound crazy but, here goes..
I had a little brother who did the same thing to my younger sister. My mom was fed up with it but one day he brought blood on my sisters arm. My mom grabbed my brother and bit him on the arm and left the bite impression on him, she bit hard enough to leave it but not enough to bring blood. They say that kids don't realize the pain they are causing when they bite so if you bite them back~ they know. My brother never bit again after my mom bit him. Eye for an eye, ya know? Fight fire with fire. Good luck and I hope this didn't freak you out!