“if parent category is” conditional?

Your solution won’t work, because $ep_category->category_parent is ID (integer) and not slug (string). So this comparison doesn’t make any sense… 😉

One way to do this would be this function (from Codex):

if ( ! function_exists( 'post_is_in_descendant_category' ) ) {
    function post_is_in_descendant_category( $cats, $_post = null ) {
        foreach ( (array) $cats as $cat ) {
            // get_term_children() accepts integer ID only
            $descendants = get_term_children( (int) $cat, 'category' );
            if ( $descendants && in_category( $descendants, $_post ) )
                return true;
        }
        return false;
    }
}

After adding it to your theme, you can use it this way:

if ( in_category( 'watch-isatv' ) || post_is_in_descendant_category( 11 ) ) ...  // where 11 is 'watch-isatv' category ID