Apologies in advance for the length of this post. I know it's really long and I'm sorry if it's rambly. 
I've been skulking around the internet trying to get advice about this for a few weeks, but nothing seems to quite fit my situation. I also hope that by sharing it someone else will benefit in the future.
My AIBU dilemma is how much I'm prepared to help my in-laws, specifically my FIL, but to understand how we arrived here I need to give some history.
It all starts just over 3 years ago when my MIL left my FIL due to his controlling behaviour. To start the separation she moved out of the family home and began staying at the flat my DH (then boyfriend) and his brother were living in.
My DH (32) and BIL (36) had bought the flat together the summer before and at xmas my BIL decided he was going to move to Australia in the autumn, it was just for a year for his work. My DH then asked me to move in to replace BIL - this suited us both as we had already been discussing living together. A couple of months after we agreed this my MIL (65) left my FIL (66), it was around easter time, and started staying at the flat. Before then, she had been staying quite frequently at my BIL's invitation (it might be worth noting that he's single/divorced), but she always went home at the end of the week/weekend. My DH wasn't that keen on having her there so much, but BIL and DH never discussed it. Non-communication is a recurring theme in their family.
There are only two smallish bedrooms and a single open plan living area, so rather than sleep on the sofa my MIL alternated sharing the double bed with each son every couple of nights. During this time my DH/Boyf spent a lot of time at my shared house so that he didn't have to share a bed with his mum. During the first few months after she left, my FIL stepped up his controlling behaviour by writing horrible anonymous letters to my MIL's friends, hanging around outside the flat trying to see my MIL and leaving her abusive voicemails etc. At their parents' request, DH, BIL and their eldest brother (40) tried to mediate the separation, there were family meetings which ended in arguments, they avoided communications via solictors because of the expense. MIL often directed the brothers to take messages to FIL and fetch her items from the family home where FIL Was living, but she would not communicate with him directly. FIL is very old fashioned and believes his wife should obey him and not have friends that he hasn't chosen for her. MIL said she wants to be independent and life her life without his control. (tbh, I'm not sure she knows how to be independent, espesh after 40 years of being married to him)
After a few months things calmed down a bit, but 3 more months went by in this casual arrangement, with my MIL spending morning, noon and night watching loud trashy TV in the shared living space and getting more and more comfortable in the flat. Whenever I was at their flat whilst MIL lived there DH was very highly strung. He would not touch or hug me, would not even sit next to me on the sofa. He got very frustrated and snappy when she would not clean up properly after herself. I thought he was going to explode when he found her using metal spoons to scrape his expensive non-stick saucepans. The poor guy was very stressed :( BIL by contrast seemed to tolerate it well as MIL did all his laundry and cooking as he works long hours (he's a surgeon). On a slightly related tangent, BIL had asked DH to do some of his laundry and cooking when they first moved in together, but DH told him that categorically was not going to happen. The date that BIL was moving to Australia was fast approaching and at that point I told DH that I wasn't going to move in with him and his mother, so unless she found herself something more permanent, I needed to find a new place to live as I was scheduled to move out of my house when BIL left.
Again, no one in the family had discussed MIL's plans for the future or what was happening with the separation. All we knew is that MIL would not divorce FIL because it was shameful. She would not respond to questions about where she would be in 6/12 months, only that she wanted FIL to visit a psycologist. FIL did oblige, but didn't like psycotherapy so only attended a few sessions. As the move out date approached, both BILs started applying pressure to DH that MIL should move in permanently and take BIL's room until MIL and FIL had sorted things out. He said no, I don't want to live with my mother, I want to live with my girlfriend, just as BIL gets to move to Australia as he wishes and other BIL gets to live with his wife and 2 children.
Both BILs applied some more pressure (MIL was silent) but stopped when DH pointed out that MIL couldn't afford to pay half the mortgage, and neither BIL wanted to have to pay this for her. It might also be worth noting that during the separation eldest BIL didn't have his mother to stay for even a single night.
DH then asked his MIL to start making plans for a permanent place to stay because she couldn't stay at the flat after BIL had left. MIL didn't take it well and cried a lot. BIL laid into DH saying what a disgrace he was and how disgusted he was with DH's selfish behaviour. BIL said that if he was DH, he would have taken out a loan to pay the mortgage if that's what it would have taken to have MIL stay there. BIL isn't great with money and already owed DH money for shared bills that he had not paid. DH felt guilty about asking her to leave, but feels it was a reasonable thing to do.
MIL and FIL also own a house abroad, so DH suggested MIL go there until she had decided what to do with herself. She did this for a few weeks, admittedly this was a bad idea as FIL found out and followed her there and began harassing her again. She then went to Australia to stay with BIL for a few weeks, then came back and stayed in various friends' spare rooms for another 5 months. She occasionally came to stay with us at the weekend, sometimes because her friends had asked her to give them some space. When she was with us she used to make comments about how her friends would wait on her, bring her doughnuts, tea, cakes and biscuits etc. Things that we did not do when she was staying with us - we don't keep these things in our home and neither should she because she has type II diabetes! Winter came and went and in the spring, she started proceedings for a non-molestation order against FIL, after this was granted she got a 6 month occupation order for the house. A year after she had left him, she moved back into the family home and FIL moved into a bedsit in a nearby town.
Fast forward 2 years, this is how they have been ever since. DH and I got married a year after I moved in, we didn't invite any of his family or mine, we just had one witness each. Partly because we're saving money for a house deposit, and partly because we knew his MIL and FIL would argue if they both attended. It was easier to not invite anyone than have to pick favourites and/or deal with all their bullsh*t.
DH didn't see his father for the best part of a year, he was ashamed and disappointed at how he had behaved, but felt sorry for FIL as he was alone and had lost many friends and family as a result of this separation. FIL lives in a single room in a mouldy old house. He could afford to live somewhere nicer, but chooses not to. We think it's because he likes the company of his housemates. MIL and FIL have a lot of debt so FIL keeps his money in cash so that their creditors are not aware of it.
MIL still refuses to communicate with FIL and FIL has asked (via the sons) for her to sell both houses and split the assets so he can move on. She doesn't want to because she likes that house (4-bed detached house) and wants to stay living there. The last time FIL asked for this was a few months ago. As far as we know MIL has still not responded to this request.
MIL and BIL are still very close although BIL has a girlfriend now (25). BIL often pays for MIL to do and have nice things, like getting taxis instead of using her bus pass. MIL is living off a state pension, so isn't very wealthy and is still paying down the debts that she shares with FIL. DH has offered her money for practical things like roof repairs and a cleaner, but she would prefer him to pay for her holidays like BIL does and has asked for money for this. She explained that BIL bought the flights for a holiday to India, so perhaps DH could pay for the accomodation and other BIL for spending money. We haven't been on holiday nor had a honeymoon as we're still saving for a house deposit.
If you're still reading now then bravo, cos this is where it gets to the sticky AIBU....
Recently, FIL has been very ill and was diagnosed with cancer a few weeks ago. They think it's operable and hopefully will lead to a good prognosis without the need for chemotherapy. DH and the other brothers are upset that their father is ill and living in a horrible room.
Although the occupation order has long sinced expired, MIL will not have him back in the family house, nor will she sell it or the other house, nor do an equity release. She has said that if one of the sons moves back into the family house then FIL can stay there whilst he is ill. MIL suggested that if one of the brothers will not move back in and be there 24hrs a day (nevermind that they all have jobs), they should get FIL a live-in carer instead. This costs around £1000 per week. BIL agreed to this, but DH and eldest BIL think this is a ludicrous expense. When they pushed back, MIL sent them all a long message saying how they owed MIL and FIL to pay for this because they spent money on bringing them up as kids, clothing and feeding them. How she was ashamed by how heartless they were, and how they should not expect FIL to pay any of his savings towards this 24hr carer if they truly cared about him. She repeated all the stuff BIL had said about DH's behaviour when he asked her to move out of the flat. When I read it I was so angry, who has children with the expectation that they will "pay them back" when they get old. Eldest BIL and DH refused further and said that this was an attempt to guilt trip them into doing what she wanted and it wouldn't work. FIL doesn't actually need a 24hr carer, but MIL wants one as she does not want to be alone with FIL.
Unfortunately, BIL had already told FIL he was allowed back in the house, but since DH and eldest BIL refused to pay for a live in carer, he can't. FIL is angry with MIL and has responded by telling his brother (uncle in law/UIL) to permanently move into MIL and FIL's house abroad and threatening to disinherit MIL from everything and anything. He called DH and told him to arrange for a solicitor to draw up FIL's will removing MIL from inheriting anything if he dies. DH refused as he didn't feel comfortable doing this to his mother, especially as she would still be left with their joint debts. Needless to say, my FIL's behaviour still isn't great. I can see why MIL does not want to live with him.
Despite all this crap, DH has asked if his father could come to stay with us whilst he is recovering from surgery. This could be a week, this could be 2 weeks, it could be longer. It would depend on how ill he is and how guilty DH would feel about asking him to leave. We have a small 2 bed flat, one bedroom is ours and the other is a dressing room with a mattress on the floor for the occasional guest. This is where we would sleep if FIL moved in. I work very close to home and often arrive back 2/3 hours before DH does. DH has just stared a company and is working long hours to get things off the ground. As a consequence I often do the vast majority of the cooking and grocery shopping. Also, my husband doesn't drive, so if FIL needed taking anywhere that would fall to me too.
AIBU to say that I do not want my FIL to stay with us? I understand that he's ill, but my MIL and FIL have created this shitty mess and have had years to sort this out, but haven't. As a consequence this is creating arguments between me and my husband. I love him and clearly want to support him whilst his Dad is ill, but I have to draw the line somewhere. We've only been married a year ffs, still haven't bought our first house. DH's family walk all over him like a mat and he feels guilty if he doesn't do their bidding. He says he likes it when I force him to stand up for himself to them (i.e. not agreeing to pay for a 24hr carer or his mother's holiday wishes), but where do I draw the line on a man with cancer?
I knew his family were crazy before I married him, but he's such a lovely guy. 
He has a heart of gold, he's a great friend, but they totally take take advantage of him. ARRRGGGGGHHH 
How did I end up with these crazy people in my life?!?!??!!?!??!!?!?!!!?!?!?!?!?!?!?!!
Thanks for reading this. All opinions/comments/suggestions welcomed :)