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 secret of Chakravyuha, as it was 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 to enter the Chakravyuha but not to exit in so that the child will die in Chakravyuha and Shri Krishna can get rid of the curse.

and also during the battle (Mahbharata), 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 who were not in direct relation with him.