Oh dear, my thoughts are with you, your FIL and your family. My dd (6) has leukaemia and she has just finished the first year of a 2-year chemo. From what I understand, childhood leukaemia is far more treatable than adult leukaemia :(
I am not really an expert on adult leukaemia. But my own experience is Dh's uncle, who was in his late 40s or early 50s (i was not close to him) had leukaemia about 4 years ago. He went through chemo, and went into remission, but then went on holiday overseas to celebrate and caught an antibiotic-resistant type of pneumonia and did not make it :(
Does your mil not follow him to your fil's appointments? Perhaps it's time that someone sat down with your FIL and talked about it? Or followed him to one of his appointments to chat with him and his Dr together?
From my experience, there are many things that you can do to help. For me personally, even just strong moral support from the family is so important to keep me going, so I have the strength to continue taking DD to hospital (which is very far from home) everyday, while working full-time at the same time. The most important thing is to be strong and be there for them in their time of need. I find it is very hurtful if people ignore our situation or pretend everything is hunky dory. Which it is not. I don't mean you have to treat them like they're dying, but just show respect for them and their situation, and make clear that you are always there for them, and ask how the treatment is going, and what they need you to help them with. I get particularly annoyed when people who know exactly our situation, ask me in an automatic cheery voice how I am. I mean, how the heck do you think I am?? It would be much more meaningful to ask how the treatment is going, if dd's suffering any side effects, if she needs anything to help lift her spirits up, if we need any help in anything, or even how I am coping with it all!!
Try not to take no for an answer if you ask FIL or MIL if they need any help, they may be shy to ask, or to specify what exactly they need your help with. For us, it's all the little daily things that I just don't have the time or opportunity to do, like pay bills or get groceries (hard to go out to crowded supermarkets as dd's immunity is very low both due to leukaemia and due to the chemo and where we are, online grocery shopping is not really available much),or get the car serviced, or clean the house after we've been to the hospital for chemo(since her immunity is low, we need to ensure her surroundings are always clean) or help bring some meals once in a while, or keep them company while they're in hospital, that sort of thing. Your FIL may need help in other areas, obviously, but these are just to give you some idea...
Anyway, this is a very good link that talks about how you can help. Hope it is useful for you too.
www.acor.org/ped-onc/cfissues/dodont.html
HTH