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!