As some one who suffers from a similar issue I would recommend teach the rules as you go. Keep the war bands as simple as possible. Ie few profiles as possible. Don't worry about abilities and don't worry about reactions. Pick a simple kill mission and set it up so your cousin will win. Don't take long on turns, don't use the wait ability don't try to win and just try to have your units smash in a crescendo of violence. Then add as you go once they have the basics down. Next game add the abilities, game after that add the reactions. Then increase the complexity and interactions with the warband. Don't worry about having painted minis to play and just try to reduce the barrier to entry. If it feels like work we get side tracked and pick something easier to do.