Final answer:
Winston and Vincent likely stop playing chess with Waverly because they are frustrated by repeatedly losing to her, as she becomes highly skilled at the game.
Step-by-step explanation:
Winston and Vincent most likely stop playing chess with Waverly because they are tired of always losing to her. While the rules of chess are constant and provide each player with a known set of options, the actual gameplay depends on the decisions that players make during the game. As Waverly becomes more adept at chess, it becomes increasingly clear that her brothers cannot beat her, which is likely the reason they cease playing with her.