I would be a bit warey (Sp?) of sharing a tooth brush.
From what I read, it is how often you introduce the bacteria into your little ones mouth. So at first, they can fight it off, but eventually they wont.
We used to let our daughter do the same- it also became a battle just in that she would brush our teeth, and then refuse to brush her own.
I suggest putting names on toothbrushes, and letting her use Your toothbrush to brush yours, hers to brush dollys, etc. She is begining a phase of awareness of whats hers and whats not- so you may be able to capatilize on that.
Once a week, I put all of our toothbrushes into our own cups with hydrogen peroxide. It doesnt kill the strep mutans, but I figure it gets rid of some of the bacteria. They sit over night, and I wash them out. You can use preoxide as a mouth wash, so I figure it is not a bad toothbrush cleaner.
Anyhow- good luck.