D..
She is too young to share, but she's not too young to learn that she may not take toys away from others, even if it IS her brother. If you put her in Mother's Morning Out, they would not allow it.
When you have a babysitter one morning, go observe a two year old classroom at a nice daycare or MMO and see how they deal with it. When you see that other two year olds are not allowed to, and how the teachers handle it, this will help.
Julie S. is right in what she is saying, and those who tell you to pick her up and remove her from the play area are too. If you don't show her that she can't, how is she supposed to learn?