A person needs to have parents before you can add siblings. So, add the parents. If you don't have the parents info you can just click on the [OK] button without entering any data. You'll get "Mother of ____" and "Father of ____". Now you should be able to add a brother or sister. You can edit the parents info later if you get it.
FYI. I recommend getting a free account on the Family Search website. It's a great resource for genealogy research.
