I have exacetly the SAME problem! My two year old daughter will sit on, step on, or lay on top of her 5 month old brother! He is always in his swing or in the play pen because his sister will manhandle him!
I find that it's not because she is trying to hurt him or even compete with him, but she just doesn't realize that she CAN hurt him because she is much bigger than he is so I have to explain "soft touches" 20 times per day. One day I will have to come up with an invisible force field to put around infants to deter dogs, cats, and siblings! ;-)