For twenty years, Baha'u'llah trained and protected Subh-i-Azal

For twenty years, Baha'u'llah exerted intense "effort" in teaching, discipline, cultivation, and preservation of his brother, Subh-i-Azal. He describes nurturing this soul by teaching him the "words of the Merciful One", comparing this detailed effort to how the people of India teach their birds. Baha'u'llah explained that this guidance was so basic that it was like teaching children the alphabet. Furthermore, Baha'u'llah affirmed that for twenty years, He tried to protect and preserve this individual, despite having full knowledge of what was hidden within his heart. However, Baha'u'llah notes that despite this extensive nurturing, this individual turned against Him and became an active adversary.

(Extracted from Lawh-i-Sarraj of Baha'u'llah)

https://oceanoflights.org/bahaullah-st-201-fa/