Hi this is a really common problem and can be done through frustration, teething or for attention. I would suggest that when she bites you ignore her completely and focus all the attention on her brother and really go over the top on him whilst making sure she is getting no attention at all. When she is playing nicely again, really praise her and say how happy you are. Try and encourage them both to play nicely together for short periods and when they do really praise them and say how nice it is that they are caring towards each other in an age appropriate way. What often happens with a biter is that they soon realise how much fuss is made over doing it and they don't care if the attention is negative or not. She will grow out of it and hopefully soon realise that she gets much more attention through playing nicely. Good luck.