I received money from my deceased brother's employer and now have a 1099-NEC. How do I determine type of income? He was the employee, not me

You can enter it as the 1099-NEC and then when asked you can enter that the income was for hobby or sporadic activity.  This is not reported to the IRS, it is just telling TurboTax where on the forms to enter the income.  This will cause it to be entered as Other income  that is not subject to SE taxes but will still allow you to enter the 1099-NEC so there is not question that it was forgotten by the IRS.