hello Jules293
and welcome to TP
so sorry to hear of your mother-in-law's fall - such a cast would be tricky for anyone; no wonder her husband and son are worrying
make it clear at the hospital (there should be a discharge social worker or some similar role) that it is just not possible for your mil to be discharged to her home, in view of her dementia and the physical care she will need - don't be backward about telling them that your fil just will not be able to cope and that their home is simply not suitable for an immobile person with dementia - maybe contact the hospital PALS for some support
the 'usual' course on discharge is to put in place a re-enablement care package, which can be in the home eg 4 home care visits a day and assessments by a physio and occupational therapist OR a discharge to a care home to provide 24 hour care and assess what is to happen in the long run (you may now want to seriously consider a permanent move to a dementia care home)
if your mil will be self-funding ie she has in her own right assets over £23250 (not including their home or any of your fil's individual assets), you may want to have a look round at the local dementia care homes, even the nursing homes, and see if any are suitable - are Powers of Attorney in place, as these will make it easier for your fil to make decisions on his wife's behalf
the duty of care for your mil lies with her Local Authority Adult Services, so your fil can refuse to have any part in her care (I know this sounds harsh, it doesn't mean that he doesn't care for her, it's a way of making the LA realise that it's not in your mil's best interests to return home) - phrases such as your mil, if returned home to inadequate care, will be a 'vulnerable adult' and 'at risk' and possibly become a 'failed discharge' should get some reaction
hopefully the discharge team will realise that it isn't sensible or in your mil's best interests to return to her home, and that she needs 24 hour care in a home at least in the short term
best wishes