The plugin widget does not show in the posts page of the TwentyThirteen theme.
It does show in individual posts and pages.
I confirmed which template file it is.
It is the standard index.php template file and it does have
<?php get_sidebar(); ?>
in it, yet the widget does not show.
I was using only one widget in the sidebar. The widget is the Related posts YARPP plugin.
Once I added core widgets such as Recent posts, etc, they did show up.
So it seems to be an issue with the YARPP plugin.