1. You can learn to get along with many different personalities, which is very important.
2.Less work (when children are older) because lots of people share the load.
3.By the time you grow up and get married you ( some of you anyway) will be used to sharing a room and bed.
4. You don't have to have company over to play tag and hide-and-seek! lol
And then I thought of:
5. You can almost always find someone to talk to/play with.
Also, love that pink sweater your older sister wears? Wait a couple of years, and it can be yours! And then when you grow out of it, your younger sister can have it, and when she grows out of it your other sister.....