Quoting *Kaleb&Bella's Mommy*:" they do... my mom comes once a week and takes one for a few hours and swaps the next week to give me ... [snip!] ... usually bites when he is playing with a toy that he wants. she will try to 'trade' first but if he refuses, she bites him..."
Well that is good, now you know what causes most of it. It's frustration. You can teach her to bite the wrist rattle, or be very vigilant. This is going to sound weird but when mine would start fighting at that age, I got a coffee can and filled it with dried beans. When one would go on the attack, I would shake it really hard. It either scared him so he didn't do it, or distracted him long enough that I could redirect him. (I learned this from a dog training class LOL)
I know how hard it is to be that vigilant though! We also put a play yard in our house and if one was being particularly aggressive, I would put him in there if I needed to go to the bathroom or make lunch or something. He'd have toys in there and just play.