Sorry for your loss,
As we are unable to see your father-in-law's tax return and understand the full situation, it's difficult to understand why you are being asked this question.
You may have entered some information about the Disability Tax Credit (DTC) and that is the reason you are being asked about it. If you are referring to the Canada Pension Plan (CPP) death benefit shown on a T4A(P) slip, if you are being asked for the number of disability months, it could be because you have entered the death benefit in the wrong box on the T4A(P) screen.
Please let us know if either of these situations applies, and we can guide you through how to correctly make the entries.