My ex was a children's librarian. Four or five are good ages. Not sure where you're located but if you sign them up you'll be liable for any fines or damaged books. Some systems in the name of equity and the fact that it often cost more to process the fine than what the fine brings in, have moved towards fine free systems. You just get charged cost of the item. Could be sent to collections.
Not sure if yoir bro/sis are responsible but something to think about.