I can see this happening in the near future with my mother. She constantly complains that she “doesn’t know what she’s going to do when the taxes are due”. Unlike some other posters, there’s no way I can afford to chip in some cash each month. 
I’m Typo Knig’s wife.
Answering some of the questions: they have no other income other than Social Security. Without going into a lot of details, Father-in-law was underemployed pretty much all his life, and has spent a fair bit of it dreaming about something he “had in the works” that was going to fix it all. Mother-in-law worked, but never made big bucks. And there were a lot of bad financial decisions at various points. So there were never any savings, and inheritances have all been eaten up through overspending, and also through just bad luck (mother-in-law was seriously ill a couple years ago). So at this point, their sole income is Social Security. Looking at my own SS estimate, and knowing my earnings history vs. theirs… they aren’t raking in the big bucks.
They also have a history of denying problems when we kids have flat out asked them. Like when MIL was ill. Like a year before that when we asked them what their financial plans were. Like several months ago when sister-in-law asked them.
The condo is underwater (financially, not literally!). Florida is a recourse state, which means the bank could go after them for the difference between the value of the place and the mortgage amount. Of course, they have no assets beyond home furnishings and a few heirlooms, so there’s nothing for the bank to seize and I presume the bankruptcy process would protect at least some of those; I think they’re going Chapter 7, and they’d get to keep about 3,000 dollars worth of personal property.
They have a decent car (which we paid the down payment for); fortunately that was leased vs. purchased. Knowing what I know now, that is probably best because I assume it can’t be seized. I’m generally against leasing, but in their case (as sister-in-law mentioned at the time, 4 years from now they may not even need a car).
Seconding this. You’ve mentioned that you would consider any money given to them a gift, not a loan - that’s a good start. I would rather give someone money than co-sign for anything - frankly, I wouldn’t co-sign for anything, with anyone, even family. I’ve watched too much “Judge Judy.” 
I think that is the first step. If your parents’ financial problems are going to land on your (and your siblings’) doorsteps, then you have a right to know all the details of the problem. Maybe if you posed it like that, they would understand that they have to come clean and work with you guys.
Since income isn’t likely to go up, expenses have to go down. Like others have said, maybe a cheaper apartment or living in a cheaper part of the world would either make it possible or almost possible, with you and your siblings making up a bit of difference.
I’m sorry you’re in this situation, too - and I imagine your parents aren’t feeling too great about it, either.