Sorry for your situation, my own FIL recently passed away from cancer.
It sounds like your FIL and wife have failed to communicate about these issues thus far, which is understandable. She's facing the loss of a parent, while he's facing the end of his life. These aren't easy things for a lot of people to talk about, but the fact is when someone dies there is a lot of business to take care of. Sooner or later you will have to deal with these things, and it is much better to at least get some shit in order before you are forced to deal with a million questions in the immediate aftermath of their passing away, which sucks enough.
If a conversation is a possibility, you should at the very least ask him about his wishes in the event of life sustaining interventions, or if he wants that at all, so you and your wife can honor his wishes. If he is able and willing to discuss that, then you can start to ask more questions about what his wishes are. Maybe he prefers cremation, maybe he's purchased a plot somewhere. Maybe he can tell you where his important documents are so you can start to sort through it now. Maybe you already know where to look, and you might find a lot of answers.
If a conversation isn't a possibility, or doesn't help much, I would run a credit report and background/public record search to be prepared for what to deal with.
Here are some things you'll have to address eventually. This is all a bit morbid, but really, trying to sort through these things in the immediate aftermath of the death of a loved one is overwhelming. You might have to just figure some of these details out right now by yourself, so that in the event of his passing, you can handle those details while your wife and SIL are grieving.
1. What mortuary would you like to use?
2. Would he prefer cremation or burial?
3. How are you paying for burial/funeral costs? This can get expensive. (You might want to set some money aside for some of these costs in advance).
4. Where are his bank accounts? You'll need to call them and send a death certificate. You'll also need to do this for Social Security.
5. What other accounts/debts did he have? Report and send death certificate as well.
6. Does he have life insurance? You'll need to know where to claim and who the beneficiary is.
7. If he had debts, was there any insurance on any of them? They might be paid off if so.
8. Where would you/he prefer funeral services and burial/internment? Does he have something paid for already?
9. Is he a veteran? If so there are more benefits you should look into.
10. Does he have retirement accounts? Who is the beneficiary? (Likely wife is he is still married, even if they divorced he may have never updated this).
11. Does his estate need to go to probate? In my state, an estate worth less than a certain amount does not go to probate, so look into it.
12. Will he have more debts than assets? If so, his assets will need to be sold to pay the debts until the money is gone. Your wife is not liable for debts that are not in her name that she didn't sign for, so if you end up with collectors calling, know that it isn't her legal responsibility.
13. You will have to file his final tax. If he owes and has other debts, make sure Uncle Sam gets paid first.
The better you prepare yourself for what is to come, the better you can support your wife. It really makes a bug difference to at least be mentally prepared for the things you will have to deal with. Best of luck.