Try to get him interested in harp, I had an 8 yr old student with a 5 year old sister, she is now also taking lessons. You don’t need to formally teach the sibling but give him a chance to try the harp then he might be more interested during lessons and have the older student play “for the sibling”. The older student can also go home and teach the younger what he/she learned in lesson that day, I think if someone is stuck sitting in the lesson they might as well be involved in some way. Just an idea!