It's only since 2017 that the other expenses have not been allowed as a miscellaneous deduction on Schedule A.  So it might not have made much difference, until then. 

Sounds like a family partnership where Mom and Dad got a discount on rent.  But was what they were paying, really below FMV taking into account that they were good tenants, unlikely to move out, while keeping an eye on the rest of the property?  

If the returns are audited, concede the issue.  That might mean going back three years.  Otherwise, there is no law requiring amended returns, absent fraud.  Let sleeping dogs lie.  Leave the can of worms unopened.  Or is one of the kids now running for office and wants to disclose his returns, to an opponent who is a CPA?