What should I do to make my sister retract her prayer "May Allah not grant you success," and should she seek forgiveness, or should I pray to Allah not to answer her?

Your sister's prayer for your lack of success is impermissible, and she would be sinful for it if done intentionally. It will not be answered because there is an impediment, which is praying for something sinful; as the Prophet, peace and blessings be upon him, said: "The supplication of any one of you will be answered as long as he does not pray for something sinful or for the severance of kinship ties." She must offer sincere repentance and resolve not to repeat such an act.