sidebar_admin_setup is an action triggered early on when editing sidebar widgets. Generally speaking, this action is triggered after widgets are set up (globals $wp_registered_widgets and $wp_registered_widget_controls are defined) but before other actions on the widgets are taken.
wp-admin/widgets.php since 2.2wp-admin/admin-ajax.php in 2.8wp-admin/admin-ajax.php to wp-admin/includes/ajax-actions.php in 3.4Triggered in wp-admin/widgets.php and by wp_ajax_save_widget in wp-admin/includes/ajax-actions.php.