G'day GeorgesA,
If they are doing you a favour by looking after the house well, then you'd already returned the favour by allowing them a lower rent - so you're even.
But then, time goes by - at the very least, I would suggest applying a CPI increase each year they are in there. They will still be getting a low rent (relatively speaking) and you will still be having "family" in there looking after the place - but keeping the rents at a more realistic level.
If they've already been there for two years, look at upgrading the rent by the amount of the last two CPI increases.
It's not up to you to subsidise your relatives' living expenses, surely. Do you think THEY would expect this?
Regards,