I heard that while Kamsa was dying, he cursed Shri Krishna that he (Shri Krishna) will also die in the hands of his nephew.
Shri Krishna had only one sister i.e Subhadra. So, while she was pregnant, Shri Krishna was telling her the secret of Chakravyuha, as it was a boring topic for her, she slept soon but the child inside her womb was listening to Shri Krishna's explanation.
The child was acknowledging that he is listening to the explanation by doing hmm sound frequently.
I heard that:
Shri Krishna told the child about how to enter the Chakravyuha but not how to exit it so that the child will die in Chakravyuha and Shri Krishna can get rid of the curse. But some other sources say that Arjuna was explaining the Chakravyuha to Subhadra, when he explained the entering of Chakravyuha, Shri Krishna interrupted him and took him away.
And also during the battle (Mahabharata), Shri Krishna as a charioteer diverted Arjuna's (who is aware of Chakravyuha) Chariot far from the Chakravyuha when Abhimanyu was entering in to it.
Is this true?
PS: There were other nephews of Shri Krishna known as upa-pandavas. Though they weren't in direct relation with Shri Krishna, they died on the night of the end of Mahabharata war.