Sub-pages

I posted this earlier today but it doesn’t show up anywhere that I can find. So if this actually is a double posting, my apologies.

I am trying to create a website (not a blog) using WordPress as a CMS. In the sidebar I want to be able to list the sub-pages of the currently showing page. When a user clicks on a sub-page link I still want the same sup-pages (they would be sibling pages at this point to the displayed page) to show in the sidebar.

I could hardcode this action into the template but I may hand over the maintenance of the site over to someone else eventually and I would like it to be as automated as possible.