there is a court case Bindseil TC Memo 1983-411) where becuase the property was rented to the taxpayer's parents who were expected to take "unusually" good care of the property and certain other expenses were avoided, fair rental in this case was found to be 80% of market. However, in Jackson TC Memo 1999-226, also rented by the taxpayers to their parents, below market rent could not be justified, so the income was taxable, but expenses, except for those that would be on Schedule A weren't deductible.
