Agree that even without a newborn I'd be very wary of having both PIL to stay in these circumstances, especially if dh was unwilling to take any time off but expected me to be carer for both of them (whether that was just being household slave or extended to personal care for one or both of them).
With a newborn some of the things that saved my sanity were being able to do what I wanted when I wanted (within boundaries of looking after ds obviously!) so if I didn't end up getting dressed then who cares. Or if I slept whenever ds had a nap and left household chores - great. And having control of the sky box and remote control so that during the day if I was just sitting feeding ds or feeling doxy - I could watch what I wanted. It sounds petty if someone is suffering from cancer but you won't ever get that year back and if I'd had to spend it getting even more tired, watching what they wanted on tv, always needing to get dressed and having to do extra work to ensure they were ok rather than being able to crash and put myself and ds first it would have been very difficult.
Dh was very ill around the time ds ws born and so at home and needed caring for. He was asleep a lot of the time but even so I couldn't put ds's needs first, I had to look after dh. Even little things like when ds woke for a feed in the night my first reaction was to check dh was still alive, not to reach out for ds. It definitely changed that time and obviously I don't begrudge it as it was dh. But for a PIL, neither of whom I'm close to, it would have been a massive struggle.
How well do you cope with PIL visits normally? And how do they? Do you want to throw them out before they've notched up 24 hours? Do they expect life to revolve around their habits and not fit in with yours? Do they help out (and cause chaos!) or expect to be waited on, hand and foot?
Other things I'd want sorting out:
If there are conflicting demands, who gets priority? eg if you are expected to take PIL to all chemo appointments then what happens if it coincides with the baby's jab appointments? Or with the meet up that your nct group is having that you really want to go to, to see all the babies and catch up with everyone? Or watch your favourite programme but it clashes with theirs - and neither of you can stand each other's choice? Or you're feeding the baby and they want you to clean their room? Or they always want to hold the baby or start to wean at 6 weeks because they did and so start to do it when you're not around to see. Would they expect to invite friend's around and make you feel like your house isn't your own? Each one in itself is quite minor and petty but if you are the one that is expected to capitulate each time then it could be miserable for you.
And later on - what if you want to go back to work but dg doesn't want you to as you wouldn't be able to care for PIL? Would they want to babysit the baby if you went to work and you wanted to send it to nursery instead?
And most importantly - What would happen when the ill PIL died - would the other one expect to live with you forever more? Or what if the cancer went into remission - would they still stay with you?
Sorry for so many more questions but the fact that dh isn't willing to do any of the caring for himself but seems to think that you are at home on maternity leave with nothing to do so he can dictate that you will become carers for them both long term... That would irk me and sit very badly to be honest. Hell, the way I felf with a.newborn - I would feel hard done by if he had expected his parents to come for a week of respite, let alone forever!