My sister has three kids too - two boys and one girl. Our kids are all basically the same age, ranging from 7 to 3. We both make an effort to get the cousins together as often as we can throughout the year. With a 12 hour drive or expensive plane tickets keeping us apart, it is hard to do, but we manage it about three times a year. It is worth the money, time and extra work - these kids love spending time together.
It is like the months since they last saw each other disappear. They play, play and play. We really have to do little to intervene - they include everyone and share the toys most of the time. And it helps that they all enjoy boxing, wrestling and Star Wars!