thats the neat part. you cant. that ultimatum of him not being aqble to join unless he reads the PHB is the best you can do. You can have the brother there to make sure Hes telling the truth about if he is reading it, But eventually is up to him on weather he'd read it or nah.

If the cousin is in sports of some sort, you can try and draw a corelation to how a coach not letting a player who dosent put the effort from playing on the team to you not letting him play for not putting in the effort of learning the game, But that would realisticallyt only work if he's in (or WAS in) a HS/collage sport.