My DGran went into step down care after all of her knees and hips being replaced over the years. (All 4 joints, the last at aged 93). There was no one at home with her on a daily basis, and no one near enough to give the level of additional care that would be required for those few weeks.
DGran loved it. Company, decent food, nice benches to sit on in a flat garden for walks, and they didn't annoy her making her go to bed and turn out lights too early at night or get up too early in the mornings like in hospital (her hours were more like 11am to 3am, hosp tended towards 7am to 9.30pm).
And afterwards, she came home fit and healthy, to her own home. There were carers coming in once a day for another week or so. And by the last two, she was already on a system where "the girl" came in for a couple of hours once a week to help her clean the house.
DGran had another 2.5 good years at home, another 8 months struggling badly with daily carers calling (family did what we could but between FT working and distance, there was not a daily option), and her last 6 months were in hospital and care home wing of hospital.
I agree that SIL should be able to go on her holiday.
I also agree that looking into care home option is probably what's needed here. It is absolutely NOT an indication that DMIL cannot cope or needs to leave her home long term. It is a period of recuperation after the operation and will help her to recover BETTER because they will see she's looked after and also that she's getting her exercises done, but also that she's busy and active and not moping. And they will also be much more likely to have suitable aids (in terms of accessible baths/showers, walking aids that help her move along but also different forms so she can go from lots of help to very little in proper steps) and be able to sort out meds etc better too (and not just meds but have ways of dealing with pain like massaging, or aromatherapy to relax, or other things that can take the mind off lower level pain, or supportive chairs to relieve pressure and make it easier....)
Which would probably, in the end, mean a faster and better recovery than going straight home and struggling through it with minimal support.