Unless you have set it to private, your tree is public by default. You can check your settings under tree pages by clicking tree settings and selecting privacy settings.
When you invite people to your tree you have the option of allowing them to see living people. Go to tree pages and select tree overview. On the bottom right hand side where it says people you've invited, click on manage invitees.
One way to find siblings is to look for the parents in the census. As far as vital records go it all depends on the state. Every state has their own laws. For example in Massachusetts vital records are public and easy to access. In other states a person might have to be dead 50 years before it becomes public.