I keep trying to convince my parents to sell it. They're spending a lot of money on it for property taxes and HOA fees (it's a condo unit) but they simply don't care enough. On the bright side, it's in a town that hasn't yet been swept up by the housing crisis. Only a matter of time though.
If that third house had some form of tax penalty associated with it being unused/empty they might bother to sell it.
But the housing crisis isn't perpetuated by empty homes in general; empty homes exist because there's an investment aspect of it. If the grandma house was losing at IRS depreciation rates, it'd likely be sold (though a storage unit might cost a similar amount).