Since I have been in the consulting business for 32 years and I have placed all my earnings in the Schedule C and think that my co-executor income should be handled the same way.
This is directly from Publication 559 from the IRS
"All personal representatives must include fees paid to them from an estate in their gross income. If you aren't in the trade or business of being an executor (for instance, you are the executor of a friend's or relative's estate), report these fees on your Form 1040, line 21. If you are in the trade or business of being an executor, report fees received from the estate as self-employment income on Schedule C, Profit or Loss From Business; or Schedule C-EZ, Net Profit From Business, of your Form 1040."
This is directly from Publication 559 from the IRS
"All personal representatives must include fees paid to them from an estate in their gross income. If you aren't in the trade or business of being an executor (for instance, you are the executor of a friend's or relative's estate), report these fees on your Form 1040, line 21. If you are in the trade or business of being an executor, report fees received from the estate as self-employment income on Schedule C, Profit or Loss From Business; or Schedule C-EZ, Net Profit From Business, of your Form 1040."
Line 21 is the line number for years prior to 2018...I'm not sure what the new 1040 Line number for "other income" is for 2018 since they've changed all the forms.