It's normal for sibling battles to heat up once the younger child is crawling and getting into the older one's stuff; the first step to avoiding these fights is prevention. While it's important for your older kid to learn to share, it's also reasonable for her to have some toys that are off-limits to the baby (especially those that have small pieces and might pose a choking hazard). These she can play with on her own in her room, when the baby's sleeping, or in places the baby can't reach, like up on a table or bed. It's okay to allow your daughter to not always play with her little brother. They both need to have the opportunity to play by themselves without worrying that the other sibling will take over and "mess things up."
If a fight breaks out, tell your toddler that you understand why she's upset: "You get so mad when your brother wants to play with your toys. I know it's hard to share." Next, help her empathize with her brother: "He just wants to explore and play with you. He's interested in what you're doing and doesn't mean to make you mad." Feeling understood and being able to put themselves in others' shoes help children move on to the third step -- finding solutions.
Brainstorm with your daughter ways to work it out. The more involved she is in solving the problem, the more likely she'll want to carry it out. For example, when your son reaches out and grabs the ball she's bouncing, help your daughter find another ball for him to play with. Help her think about words she can use to express her feelings, such as: "Please don't take my doll." This teachable moment lets your child know that you believe she can solve her own challenges.
Also, help your daughter think of things her brother can do by her side while she's playing. For example, you can read stories to your son while she builds her block tower. This way they're enjoying one another's company without getting in each other's way. Then you might ask for your daughter's permission to allow the baby to let loose and knock down all the blocks.
Finally, look for ways your children can actively have fun together. You can give the baby maracas to shake while your daughter plays the xylophone. In the car, the two of them can play peekaboo or make funny faces at each other. Later they can hide in blanket "tents" draped over furniture. Soon, your daughter will begin to view her brother as a playmate and partner in crime instead of a pain.
Originally published in American Baby magazine, October 2006.