Sibling rivalry

My daughter has started crawling forward. She's mobile and a little explorer. It's wonderful.. except that she's getting into more toys and my son is not pleased with this development.

I've had to start punishing him (with 30 second time-outs in the corner) because he started pushing her over. The first day I started this, he must have pushed her 8 times before he stopped. Since then, he's stopped pushing her, for the most part. He still does it occasionally. He is snatching toys away from her, screaming, "Noooo!", and throwing the toy across the room.

A part of me thinks that I should intervene as little as possible because I want these two to develop a friendship, but it feels wrong because my daughter is so young. There are toys that are "his" and that I don't allow her to play with (when he's in the room). I'm also trying to instill in him the concept of toys that are exclusively hers and that he shouldn't be playing with. That's a difficult concept for a 2 year old to grasp.

Well.. the toys are in front of me, so I guess he needs some attention.