Okay, Now I'm Mad

So Wednesday night I'm in the bathroom brushing my teeth when I hear Andrew start to scream. He and Beth were playing on the floor in his room. When I ran in and asked Beth what happened she said "I bit his finger really hard, and he didn't like it." I was worried that this would happen now that Beth has been introduced to biting. I am trying to figure out how to teach her that biting is wrong especially biting a helpless baby, but that there are times when she needs to defend herself.