in_widget_form detect sidebar id

The relation between sidebars and widgets is stored in the sidebars_widgets option, as a serialized array that might look like this (expanded):

Array
(
    [orphaned_widgets_1] => Array
        (
            [0] => recent-posts-2
        )

    [wp_inactive_widgets] => Array
        (
            [0] => calendar-2
        )

    [sidebar-1] => Array
        (
            [0] => recent-posts-5
            [1] => text-6
            [2] => text-2
            [3] => search-6
            [4] => text-3
        )

    [sidebar-2] => Array
        (
            [0] => text-4
        )

    [sidebar-3] => Array
        (
            [0] => text-5
            [1] => recent-posts-6
            [2] => search-7
        )

)

WordPress uses the wp_get_sidebars_widgets() function to fetch this relationship.

If we want to find in which sidebar e.g. the fifth instance of the Recent Posts widget ( recent-posts-5 ) belongs to, then we can search the array and find that it’s the sidebar-1 sidebar.

Helper function:

We could do that with a helper function like this one:

/**
 * @param string       $widget_id  Widget ID e.g. 'recent-posts-5'
 * @return string|null $sidebar_id Sidebar ID e.g. 'sidebar-1'
 */
function wpse_get_sidebar_id_from_widget_id( $widget_id )
{
    $sidebars = wp_get_sidebars_widgets();
    foreach( (array) $sidebars as $sidebar_id => $sidebar )
    {
        if( in_array( $widget_id, (array) $sidebar, true ) )
            return $sidebar_id;
    }
    return null; // not found case
}

In PHP 7.1 we could use ?string as a nullable string type.

Usage example:

$sidebar_id = wpse_get_sidebar_id_from_widget_id( 'recent-post-5' );

So within your in_widget_form callback you could try (untested):

add_action( 'in_widget_form', function( $widget_object, $return, $instance )
{
    $sidebar_id = wpse_get_sidebar_id_from_widget_id( $widget_object->id );

    if( is_string( $sidebar_id ) )
    {
        // ... do stuff ...
    }

}, 10, 3 );

Update:

This seems to have been solved similarly here: How to determine which sidebar the widget has been added to, via widget admin?.