It sounds like this was a gift, if the amount was given in cash, there are no conditions and it was not recorded on any official forms such a 1099, W2 or 1098-T Form.
Definitely, you should amend to add the royalties to your tax return.
As far as the amount of the gift, as long as it is a gift of less than $15,000, you don't have to file a gift tax return.
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