A very difficult time all round. My experience of terminally ill relatives has been that medical care doesn't leave them in pain, from which you might draw some comfort. I think all you can do is keep supporting, bring tissues for your father in law and flowers / pretty things for his wife. It won't be easy, it never is at times like this, but it will pass. see if you can do anything practical for your father in law like shopping or laundry - I'm not trying to assume any gender roles or dah-di-dah, just know from experience that the simple daily things can be greatly appreciated at times like this.
Also, please remember to look after yourself physically and mentally and stop beating yourself up, you are doing nothing but good.
Big hugs and bear up over the coming weeks.