Bite me... but not literally

Hi, my name is Nic, and my kid's a biter. I can't believe I am THAT mother with THAT child. I don't know how or why Nut does it and I don't know what to do to get it to stop. I've tried time outs, a swat or two on the bottom, and even biting her back. Nothing is working. I know the books will say it's a way for her to show her frustration since she can't fully verbalize what she's feeling, but that's not really helping ease my mind. Today she left two marks on the arm of the 3 yo I watch (prior to today that was the only person she's ever bit) AND chomped down on Bug this evening. At least - if this makes any sense - she was actually "fighting" with the one I watch. She got shoved back into a door, complete with a re-bloodied lip of the wound she got yesterday when she biffed on the sidewalk. All that was over a pink dress up shoe, by the way. Girls! It doesn't make it right but it was provoked so to speak, so I feel like maybe I can control that. Teach her a different way to show her frustration. With Bug though... I don't even know what that was all about. Sibling rivalry, maybe? Whatever it was, I need it to just stop. Any suggestions? I'm at a complete loss here. Help.