When did Arjuna know that Karna was his brother

Question

There are different stories of when Arjuna is aware that Karna is his brother. Some say it happened after the death of Karna when Krishna reveals it to the Pandavas.

But some say it happens during the Mahabharatha when Arjuna is about to kill Karna, Krishna tells him who he is about to kill is his own brother.

"You need not be afraid of killing, because all those you are going to kill have already been killed by me. They are already dead, you have only to give them the news of it through your arrows. And what I am saying is not my words, they are coming straight from the cosmos, from the beyond. From the depths of the beyond comes this information that what you see as life before you is only an appearance, it has already become extinct, it is no more alive. It is just a matter of moments when those standing across the battle lines will be dead. You are only an instrument in the hands of existence, nothing more.

So don’t think that you are going to kill them. If you think you are the doer then you are bound to be afraid. With the doer comes fear, anxiety and anguish. Every suffering, every sorrow, arises from the ego, the doer, which is a false entity. You are utterly mistaken if you think you are the doer; you are merely an instrument in the hands of the divine. Let it do what it wants to be done through you, and let go of yourself.”

So could someone tell me the correct story!

Answer 1

Kunti exposed this truth to pandavas in Stri parva:26.

That hero and great bowman, that leader of leaders of car-divisions, that warrior distinguished by every mark of heroism, who hath been slain by Arjuna in battle, that warrior whom, ye sons of Pandu, ye took forth, Suta’s child born of Radha, that hero who shone in the midst of his forces like the lord Surya himself, who battled with all of you and your followers, who looked resplendent as he commanded the vast force of the Duryodhana, who had no equal on earth for energy, that hero who preferred glory to life, that unretiring warrior firm in truth and never fatigued with exertion, was your eldest brother. Offer oblations of water unto that eldest brother of yours who was born of me by the god of day. That hero was born with a pair of earrings and clad in armour, and resembled Surya himself in splendour!’ Hearing these painful words of their mother, the Pandavas began to express their grief for Karna.

Answer 2

As per Stri Parva of Mahabharata, After the war at the time of performing last rites, Kunti tells panadavas (Arjun, Yudhishthir, Bheem, Nakul and Sahdev) to perform puja for Karna too as he was her eldest son. Yudhshthir carried out puja but becomes angry with his mother that why she kept this secret from them. He cursed her that now onwards women would not be able to keep secrets within to them.