How does one deal with sibling rivalry? My son is 9 and daughter is 5.

I guess, the best thing to do is engage your kids in more and more group activities. Make them two be a part of activities where they have to work as a team. And when they will work as a team, their bond will develop. No doubt they will fight even then, but they will bury the hatchets soonest and will learn to solve their matters on their own. They will defend each other rather than complaining. Also, they are in the years where their maturity level is still building up and they will have little fights over little things, which is all quite normal and part and parcel of growing up. These fights also make up for greta memories.