Meta + category__in query

meta-and-category-query.php

$args = array(
	'category__in' => $cat_id_for_posts,
	'meta_query' => array( 
		array( 
			'key' => $custom_field_key_show_in_sidebar,
			'value' => $custom_field_value_show_in_sidebar,
		),
	),
);