Display the id list of active widgets of same sidebar?

Despite what you say is_active_widget does work on custom widgets as well. You may just have trouble locating its ID. To help you, here’s a snippet that will show the ID of a widget on the widget admin.

add_action('in_widget_form', 'wpse202950_get_widget_id');

function wpse202950_get_widget_id($widget_instance) {
    if ($widget_instance->number=="__i__"){
        echo '<p class="widget-id-message">' . __('Save the widget to get its ID','textdomain') . '</p>';
        }
    else {
        echo '<p class="widget-id-message">' . __('The widget ID is:','textdomain') . ' ' . $widget_instance->id . '</p>';
        }
    }