The recipient should be blamed too because benefactors or regular
business partners are ought to be notified in cases like that because funds can be sent without your notice at anytime.
Both parties are to blame here. They are both responsible for the loss of those coins.
The sender (the uncle) should have asked for an address before sending to ensure the recipient has control over it and owns its private key. It’s also recommended to use a new address for each transaction and avoid address reuse for pivacy reasons.
The recipient (the nephew) is also to blame because he should have kept a backup of his wallet even if he was not going to use it anymore. Here is what Satoshi said about this:
Sigh... why delete a wallet instead of moving it aside and keeping the old copy just in case? You should never delete a wallet.