So, she just has this $600 or does she control your paycheck? Can you get an "advance" from your job? If you pay your rent (to your father) can you just keep doing that at the rate you want. It doesn't sound like he'll "fuss" and try to collect more, no matter what your mother says? Just do a thing between him and you, and bypass your mother. My father and I use to do that; had arrangements my stepmother didn't know about.

If you get paid your check from your job and she's a cosigner on your bank account or something like that; open another bank account.