Others have mentioned a will and that is a good idea. But consider putting the private key and detailed instructions in a safe deposit box and then reference that safe deposit box in the will rather than giving the private key or clues to the private key in the will.
Yes, it's a good idea. Just write all your passwords, keys detailed on a notebook and keep it safe in a place where just who will get your heritage can have access. Maybe a bank or a safe box. Or give it to your family members, so they can use your BTCs if you unfortunelly die.
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yes that is right to say, but if you are giving it to some one in advance then there is a chance that he may use his private key in advance. i think the best option will be to write the key on a paper and then keep it in a safe place and write a well in which he will give some clue to a person whom he want to use his key after his death, so this way his key will be safe.