How to make all posts in a category sticky?

You rather need a save_post hook. Check following code it marks the post sticky for the specified if it is not already.

add_action( 'save_post', 'mark_post_sticky' );
function mark_post_sticky( $post_id ) {

    if ( !wp_is_post_revision( $post_id ) && !isset($_POST['sticky']) &&
        in_category('bestcategoryever',$post_id) ) {

        $_POST['sticky'] = 'sticky';        
    }
}