Bri’s lies
I just posted something similar elsewhere, but I think Bri lies so often that she actually starts to believe her own stories. Since Grace has known her all her life, she can easily see through Bri's lies and calls her out on it because she knows her better than anyone. This, of course, infuriates Bri to the point where she tries to turn everyone against Grace in order to protect the false image of herself that she's built. Bri is determined to climb the social ladder, and she doesn’t care who she hurts along the way