@Anonymous
from my reading of the iRS rules the income is not taxable nor reportable
here is what i read as the rules
1) was personal use (this includes days rented at less than FMV) greater than 14 days or 10% of days rented?
It seems that since FMV won't be charged the answer to this question is tes
2) if 1 is yes, then was the property rented less than 15 days? again days rented at less than FMV are not rental days. so the answer to this is yes
3) since 2 is yes the IRS says treat as personal residence. do not report income or expenses. There is nothing that says treat a second residence any differently than rental property.
in fact the IRS says any day rented to anyone at less than a fair rental price is considered a personal-use day. Days rented to family members are persona-use days unless fair rent is charged (which is not the case here) and the unit is the family member's main home (IRC Sec 280A(d)(3)
