This question refers to the story Rules of The Game by Amy Tan.
Answer:
Winston and Vincent stop playing chess with Waverly because she has beaten them countless times.
Explanation:
Vincent and Winston are both Waverly's brothers. In fact, they are the ones who teach her to play chess. But there comes a time when "the student surpasses the teacher" and Waverly begins to win all the games against her brothers.
This results in her brothers getting bored of playing with her, since they cannot beat her and decide to change the game.
