What is the best way to deal with my sister who converted to Bahaism, and how can I discuss it with her? What books or figures can be beneficial in this regard?

Bābism, or Bahā’ism, is a disbelieving doctrine founded by Mirza Ali Muhammad Reza Shirazi, who claimed divinity had incarnated in him.

Among their disbelieving tenets are: That Bahā’ is the creator of all things. The belief in incarnation (ḥulūl), unification (ittiḥād), transmigration of souls (tanāsukh), and the eternity of beings. Belief in the prophethood of Buddha, Confucius, and Zoroaster. Agreement with Jews and Christians regarding the crucifixion of Christ. Esoteric interpretations of the Qur’an. The claim that the Bāb’s religion abrogates the Law of Muhammad, peace and blessings be upon him. Prohibiting jihad and carrying weapons. Prohibiting hijab, permitting temporary marriage (mut’ah), and the communal sharing of women and wealth.

You must invite your sister to repent from this disbelief and error, to pronounce the two testimonies of faith (Shahadatayn), and to disavow this false belief.