Choosing your wedding party is usually a delicate and tricky matter. It's easy to begin to worry about whose feelings you will hurt, but you've got to stop now! The entire point of a wedding party is to have people around you who love you and support your decision to get married.
One of the biggest questions people ask is: do siblings have to be included the wedding party? There is no definitive answer to this question. Generally, if you have a relatively healthy and positive relationship with your siblings, the answer is yes, you should include them. This goes for both the bride and groom. They are your family and have been there as you have grown into the person you are today.
Of course, there are always exceptions to this rule. If you have a brother or sister that is much older than you and is busy with a family of their own, you can always give them a different duty besides bridesmaid or groomsman so that they don't have to make such a big time commitment. The same goes for young siblings who are in elementary or middle school — it wouldn't be appropriate to give them such large responsibilities.
If you are truly stumped on this matter, the best thing to do is seek advice from your friends and family. This is an important day in your life and one everyone you care about should be included in one way or another!