Backend?

Sure thing, it was right there in front of me. The answer is in the readme file:

### How do I use it?

Connect taxonomies together like so:

`
/**
* Use ‘post_tag’ taxonomy as a family for categories
*/
function your_plugin_thing() {
wp_set_taxonomy_family( ‘category’, ‘post_tag’ );
}
add_action( ‘wp_register_term_families’, ‘your_plugin_thing’ );
`