So your parents are preventing two young families from starting their lives.
Seriously, a "winter escape"? Literally noone needs that. Literally noone should have that until everybody's basic needs are met.
Which won't happen with all these "storage unit" houses there are.
I don't necessarily agree that "nobody should have X until everyone has Y" is a workable general rule; no family should have two cars until everyone has one; no family should have two jobs until everyone has one.
The second home is already slightly tax disadvantaged compared to the primary residence, perhaps it should be "punished" more.
Which is why I assume the parent post mentioned 4, I would have said two or three myself (as your example shows).
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).
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.
Not that this is a good use of a perfectly fine basic housing unit, but it isn't an investment.