Negative chess players
Hohohohi888, if you lie to your cousin so he doesn't come over I doubt this will improve his attitude. The same applies to merely hinting at the problem. I would recommend discussing it with him in no unclear terms as I presume you do want his attitude to improve. However it seems some people are beyond improvement (for certain others it just takes a tremendous amount of time and varied tactics). If this seems to be the case with your cousin, if I were you I would reject playing with him while he is at your house and tell him why every time. I would not be worried about offending someone by simply rejecting to play a game of chess when the individual obviously has no problem offending you. Alternatively you could give him a dose of his own medicine. Be prepared he will probably act like he is immune (at the beginning) as he will likely understand what you are doing. This is why, in my opinion, when you decide to take this route you must give said individual a heavy dose of their own medicine; maybe over an extended period of time. However this won't necessarily bring the parties involved to a truce, it might though. It's surprising how defensive most people become when they start getting the same negative treatment they take so much pride in providing.
eat raw onions secretly in the kitchen after he asks for a game. no matter win or lose as long as you breathlessly exclaim some point about the last move he will quit coming over after three games.