My Wolverine

So, Monkey and Punky Doodles are inseperable.  They spend the majority of ALL day together and their relationship has its tense moments.  Lately though my Monkey has become Wolverine and bites and scratches Punky Doodles.  Kids will be Kids, right?  Wrong, she scratches and bites hard enough to leave marks.  How do I stop her from doing this?  She says she understands why we get upset with her when she does it.  She doesn't like it when it is done to her, but what can we do to curb this behavior.  I don't want my middle child to be a bully and I don't want my youngest to grow up hating her big mean sister, but I am at a loss.  Where in the parenting manual do you find the answers? 


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