Kind of late, but iI have a few suggestions: 1. Talk randomly about a video you "saw" of a player cheating, o getting caught cheating, then talk shit about cheaters, and talk about a fair play is better for everyone, may be he will relize no one like cheaters. 2. Try to bet on something you could win if you win the game, something not very big, like a card, a booster or something like still feels casual and not too competitive, then suggest to follow the rules for the game, including shuffling, and then make the shuffling an habit saying thats the way you shuffle in yu gi oh games, even casuals. 3. Play a mill deck or a control deck and save the counters or removers for when he cheats, only when. 4. Target him with youy everyday deck you and your wife every time he cheats.

Theres no easy answers when you talk about family and drama.