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.