Question: When did the Pandavas learn that Karna was their brother?
Answer by Romapada Swami:
This is mentioned in the Stri-Parva of Mahabharata which happens after the war is over and all the warriors (including Kauravas) are killed. The last section of this Parva describes the cremations and funeral rites for all the warriors that Yudhisthira arranges.
Relevant excerpt from the book:
Night had already fallen when all of the cremations were completed, and at that time, Maharaja Yudhisthira organized a procession to the banks of the Ganga, placing Dhritarashtra at the front. There the Kaurava ladies offered oblations of Ganga water to the departed souls of their relatives.
At this time, Kuntidevi overwhelmed by unbearable grief, all of a sudden announced, “My dear sons, your great enemy Karna was actually your elder brother, for he had been conceived by me prior to my marriage with Pandu. Therefore, all of you should go now and offer Ganga water to him for his eternal well-being.”
When the Pandavas heard this, they became practically stunned due to great shock. In a voice that was laden with intense grief, Maharaja Yudhisthira then requested, “My dear mother, please tell us just how it came to be that you had given birth to Karna before your marriage, and how he came to be known as the son of a Suta.” After hearing the entire story from Kuntidevi, Maharaja Yudhisthira called for all of Karna’s surviving relatives. The Pandvas then went with them and entered the river in order to offer the sacred water for the benefit of Karna’s departed soul.
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Immediately following this is the Santi Parva which starts with Narada Muni narrating to Yudhisthira the history of Karna’s curse by Parasurama. {Karëa had been cursed by Paraçuräma that, when he most needed it, he would not be able to remember the incantations to invoke the brahmastra. Karëa had deceived the sage into thinking he was a Brahmin in order to receive his teachings. When Paraçuräma discovered the lie, he uttered this curse.} After hearing that, the Mahabharata says the following:
Hearing about Karna only caused Maharaja Yudhisthira’s grief to become magnified. Although herself very aggrieved, Kuntidevi then urged, “My dear son, please do not lament for Karna’s death. Both myself and Surya had tried our best to persuade him to join with his real brothers, but we had failed. Since Karna had remained determined to continue his hostility toward you, even after being informed of his real identity, his death should not be lamented over.”
Actually, the grief stricken Maharaja Yudhisthira had become quite angry at his mother because her secrecy had caused him to become the unknowing slayer of his elder brother. Now, in order to give vent to his pent-up rage, Maharaja Yudhisthira uttered this curse: “From this time onward, may no woman ever again be able to keep her secret to herself!”
