well, i suppose it was wrong to read her diary, but when you think about it, people are probably more honest in diary's than anywhere. Also, if I saw a page in a diary and I saw my name, then I would be curious and read it too. That being said, I look back over some diary entries that I made when I was an aupair, and I think, 'wow, did I really think like that?!'
It sounds like she wrote some pretty uncalled for things about your family though. I suppose the only thing you can do is talk to her. Good luck!!