If I own a house in Dublin and you own a house in Galway, and we house swap, while retaining ownership.
Am I supposed to declare the notional rent?
I wouldn't have a clue Brendan, and I hazard a guess that even Revenue wouldn't have a simple answer to it either.
But in relation to the OPs first question, Yes they most definitely are required to make a tax return for the rental income
This is basic tax knowledge and the OP should know this!!
The second question regarding the rent a room scheme also I believe needs a yearly tax return to declare this non taxable income
and this is where they might get a nasty surprise if revenue have cause to investigate the declarations on the tax return,
in my opinion and I feel the majority of people would agree that the current setup the OP has with his sister would not pass the requirements that are needed to avail of the "rent a room scheme".
My work around on the other hand removes the taxable income (rent) from the equation if the sister rents a property for her brother
Is this taxable, yes I think it is through gift tax between siblings but I would think/hope (maybe naively) revenue would turn a blind eye to it in the short term but It's a long time since I've dealt with revenue so not in any way sure how they would view this arrangement
Another option is the sister rents a room in the OPs rented property and between them they occupy OPs house to satisfy insurance requirements