I'm so sorry about the illness and stress in your family! I'm hoping for peace for all of you.
My husband comes from a family that believes that nothing is truly "real" until you say it out loud. Even if something is common knowledge, they will act as if it doesn't exist, and be perfectly happy - until someone says the obvious out loud. THEN they'll all fall apart, AND be angry at whoever said something.
That person is often me. I was raised by a very pragmatic, no nonsense mom, and I'm the same way. I look at behavior that my husband's family thinks is healthy coping, and think, "What the h*** is that?" My husband has actually said to me, more than once, "Well, now I have to deal with (insert subject here.)" Honey, news flash - you'd have to deal with it whether I said anything or not! I mean, do they honestly imagine that mere human speech alters reality?
So, if it were our family in your situation, my husband would be losing his mind and coming undone not because his dad is sick, but because his dad actually spoke the possibilities out loud. He'd need lots of standard, possibly not even factual reassurances that everything will be fine. He'd rather have any bad outcome take him by surprise, so he'd be angry that his comforting mechanism of the element of surprise was gone.
If it were me, I'd be happy that I got to talk to my dad and let him say exactly what he wanted to. If something bad happened, I'd be angry and sad, but the worst of it would only last a day or two. My husband takes longer than I do to work through things. (And that's OK.)
So, what to do for your relatives depends entirely on their personalities. Some people need to talk and talk and talk about what's wrong, while others fall apart if the conversation is about what's wrong. If you don't know what your husband or MIL need, watch them. If they're happiest doing ordinary things, keeping life "normal," try to keep things as "normal" as possible. If they need a shoulder to cry on for hours at a time, let them cry.
What works for one person will annoy another. What works right this moment might not work in an hour. Everyone will be exhausted. Remember that you can't fix everything all the time, and it's not your fault if someone is sad or angry. It's normal to be both, and it's normal to occasionally feel just fine and be able to laugh. Usually, everyone will go through those stages and more.
God bless!