The transaction in question is coinbase.
I think OP have a fake wallet showing he has mined bitcoin. Once he did reindex, the transaction status changed to "0/unconfirmed, not in memory pool" which means that the transaction was neither found in the blockchain nor in OP's own mempool.
To be honest, it looks like one of the million cases where people sell "satoshi era" wallet.dat files. But OP says it was their brother's computer, so I respect their statement.