Listing pages on the same level of the tree

I’ve been grappling with a wp_list_pages issue for a while now. Here’s the problem:

On a Portfolio page, I have three sub-portfolios that I list in the sidebar. This is easy: I include pages that are sub pages of the current page. When I click on one of those subpages, I’d like to show that same list: the other pages that are sub-portfolios. I’m not sure how to do that.
In other words: List all pages that are on the same tree level. (All have the same parent). Anyone have an idea?

Thanks for your time!