How do i stop her from stealing her brother's toys?

Stealing?: Maybe the answer is to encourage the two of them to share toys with each other. I think the more you try to keep her brother's toys away from her, the more attractive they will become.

Modify behavior: Children are often possessive of toys, they often grab them from others. Praise specific sharing behaviors with a kind comment. When undesirable behaviors are seen, give a time out to the 'grabber'; if the behavior continues separate your children or give the toy a time out. Often changing the response helps improve the behavior. Try not to reward any loud protestations from either child.