S.W.
I have to agree with Jae T. in her response. Your daughter's friend was physically interupting the play, so the solution your daughter used was also physical. My mom used to say "unless there's blood or broken bones, don't come to me with it!" Several of my mom friends now use that philosophy with thier kids. There is a lot to learn from working it out among themselves. I don't think girls should always be "sweet and kind".
She also has a right to defend herself against her older brother (I had two of those myself).
My daughter started Karate classes when she was five (I am in Karate, too). I think martial arts training is a wonderful thing for all kids, but especially girls. It does not teach violence at all, but teaches kids to speak up, be aware of themselves, be confident in unexpected situations, and defend themselves if necessary. It's also a great and safe way to get out frustrations and physical energy!