Was an estate opened or was the farmland in a trust?
The document granting your brother-in-law power of attorney (POA) is invalid since a POA terminates upon the death of the principal.
If title has already vested in your spouse and his two siblings then, absent unusual circumstances, they can agree to split the income in any acceptable manner (typically, they would simply split the net income three ways).
Rental income from real estate (including farmland rented for cash) is reported on Schedule E of your 1040.
