I'm not sure how many of you have children that are old enough to participate in athletics or even music events. But, if you do...do siblings come to show support for their brother/sister?
Mine aren't old enough for that yet, but if they were, I would have them come out to support. I don't think they should be forced to every single time but being there some of the time is a good thing. In the long-run it will bring them closer.
Sometimes they come along to cheer and sometimes they don't. When they were younger, we would have to bring them to keep them from being home alone. But as when they became teens I left it up to them. Much of the time they'll go and just hang out with their friends.
Pokerdad is right about teenagers. It is like this is the time when they only wanted to mingle with their friends. Before and after teenage years, siblings usually support each other at these times.
My kids are too young to leave at home alone, so we go to games as a family. I'm sure when they get to be teens, they'll go their own ways more and just support them in other ways.