Sidebar Page Template not working -Why does it exist? Need sidebar on posts page

Hello, My friend’s site is built on a Twenty Eleven child theme as a one-column site.
She wants to have a sidebar on the Blog/Posts page so we thought that it would be as easy as choosing “Sidebar template” on the page options for her blog. It’s not.

When we selected the sidebar page template nothing changed.
What is the functionality of this page template if it does not do what is intended?

I’ve read posts that recommend editing the single.php file but I don’t currently have a copy of it in the child theme and don’t know what else I can do.

It seems to me that this should be a feature that is incorporated into the theme design without needing further php editing.

Anyone know if there is a way for me to add a sidebar to her posts page without editing the single.php file?

Any help is GREATLY appreciated!
Cheers,
Ana