The Book of Tea Series By Judy I. Lin.
For Ning, the only thing worse than losing her mother is knowing that it's her own fault. She was the one who unknowingly brewed the poison tea that killed her—the poison tea that now threatens to also take her sister, Shu. When Ning hears of a competition to find the kingdom's greatest shennong-shi—masters of the ancient and magical art of tea-making—she travels to the imperial city to compete. The winner will receive a favor from the princess, which may be Ning's only chance to save her sister's life. But between the backstabbing competitors, bloody court politics, and a mysterious (and handsome) boy with a shocking secret, Ning might actually be the one in more danger.