It is very thoughtful of you to think about how you can continue to share your family tree after you are gone. I can think of a couple of ways that you can accomplish this:
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Create a link for your nephew to edit the tree, but leave it for him in a letter in a safe deposit box, or in a will.
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Likewise, leave the username and password to him in a letter or will. If you decide to go this route, you might consider giving him a non-editing link now, so that he can ask you any questions about the tree.
Both of these are simple and reliable solutions, but the second one would be the most reliable I think.