First, try not to hold it against them that they adamantly opposed your marriage, and in fact refused to meet their future daughter-in-law. 

Then, once they're both incapacitated—once Ida is diagnosed with the kind of chronic obstructive pulmonary disease that would bring a charging rhino to its knees and Doug with the sort of dementia that causes him to ask 24 times over the course of a single lunch, "How long have I lived here, anyway?"—spend three to five years taking over their paperwork. 

Track down 50 different bank accounts, some of which have balances of $127.32, at 18 different banks. Work your way through a six-inch stack of U.S. savings bonds in the safe-deposit box. Discover that Doug and Ida hold three shares of something for which they receive a check for 12 cents every quarter. Spend a year convincing two self-reliant children of the Depression to grant you power of attorney, and then on the big day, watch them refuse to go to the lawyer's office to sign the papers. Become the kind of daughter-in-law who threaten the elderly. Then answer ten calls a day from Doug about why he's no longer getting any paperwork in the mail. 

Deal with the desolation of accompanying Doug to a dementia evaluation. Watch him fail to draw a clock face, bunching the numbers and hands together in the top left of the circle like corralled animals. Try to present a smile when he says apologetically, "I used to know how to do this." 

Sell their condo, despite their trepidation about the idea, and fudge for them the amount you receive.Laugh when you can. Laugh when Doug looks into his lap at dinner and says, "Where'd I get these pants?" Or when he calls to tell you he can't reach his mother and brother in Connecticut, and you respond that's because they've both been dead for a while now, and he answers, "So who am I talking to down there, then?" 

Figure out a way—imagine the logistics of the Normandy invasion—to get them both to your house safely for Christmas Eve dinner. This will involve backup oxygen tanks, spare tables, and something called a cabulance, as well as hauling Ida's wheelchair (with her in it) up any number of steps and through a narrow doorway. It will involve cleaning and bandaging Doug's head wounds after he falls getting out of the cabulance. It will involve wheeling Ida into the bathroom to wipe her. It will involve reminding her, once she's eaten too fast and started to choke, to breathe through her nose. This will seem beyond her. So it also will involve explaining that she should pretend she's smelling flowers. It will involve telling everyone else to keep eating while she gets the hang of that. 

Strong-arm your children into a meal at their grandparents' house at least once or twice a month, even given the impossibility of coherent conversation around that dinner table, even given that the rooms smell stale and sad and the TV is at a volume that allows neighbors to listen in. Remind your children that Doug and Ida are still their grandparents and will continue to be visited until they're not there to visit any longer. 

Have your days hijacked by all the things—aides, pills, doctors' appointments—you have to organize and supervise, and, once Ida is hospitalized for mini-strokes, stand by helplessly while she refuses any more diagnostic tests and checks herself out of the ER. Discover the despair you feel when, after you plead with her, "Do it for us," she refuses. 

Do your best to grapple with the wedge that all this stress drives between you and your husband. What daughter-in-law, would want to subject herself to such a black hole of unhappiness? What son wouldn't be sad at his wife's refusal? What daughter-in-law wouldn't note that these are two people who, even when they were healthy, were expert at making themselves miserable? What son wouldn't feel as though he owed them at least his presence? 

And when you're at your most self-pitying, try to remember that they were sometimes generous. Recall that Ida passed on her prized Persian lamb coat to you; that they gave you the down payment for your house; that they sent the kids care packages of toys. Try to remember that you can never deliver happiness, but that you can help. Try to remember that all they're doing, finally, is all that they can. Try to remember that the same is true of you.