It's entirely natural for hospital patients to want to go home but, given the circumstances, it clearly isn't advisable for your dsis to return to her home without significant changes having been made to her accomodation and a care package containing a high level of nursing care (which, ime, is beyond the capacity of most local authorities to provide let alone co-ordinate/supervise) being put in place.
Given what you have said about your dsis's physical health, the worst case scenario is that she is discharged from hospital to her home, after which event her dh is similarly discharged from his current care home and the two of them are effectively left to fend for themselves with a basic care package consisting of morning, midday, and evening visits from care workers and sporadic visits from community nurses - all of whom in the absence of a court order can only gain access to the home if the occupant(s) allow them to enter and all of whom can only perform those services which the client(s) allow.
Unfortunately, as things stand at the present time, this would seem to be what has been/is being planned for your dsis and her h and as she has full mental capacity you cannot override her wishes, nor will you be allowed to do so by her care providers.
If, as seems to be the case, you have decided not to draw further attention to the inadvisability of your dsis being returned to her home and have determined that you will stay there with her until she is 'settled', imo it is imperative for you to attend the access visit so that you can a) assess your dsis's living conditions for yourself and b) become cognisant of what improvements/additions will need to be put in place such as pressure relieving mattress, high seated chair and pressure relieving cushion/ring, commode/raised toilet seat, zimmerframe, grab rails, new washing machine etc before your dsis's return.
From what you have said today of her medical issues, it seems that your dsis is not terminally ill in the most commonly understood sense of the term, nevertheless, I agree with your expectation that she will require further in-patient care within a short time of discharge.
It occurs to me that while whatever necessary works are being carried out to her flat, your dsis could be discharged to your home and it could be put to her that this would be preferable to her being placed in a step down or intermediate facility as she could convalesce in your company and the sea air of the south coast and regain some strength before returning to London.
Your local authority will be able to provide a pressure relieving mattress and other equipment if required and should your dh demur at this proposal I would suggest that you use two words beginning with f and o give him an assurance that unless he shapes up, you'll be taking steps to ship him out and you won't be going to any such lengths to ensure his comfort should he develop a chronic medical condition.
If your dsis should require a further admission to hospital while in your care she will be taken to an establishment near you which will, at least, enable you to visit her daily or more frequently than you've been able to undertake to date.
While I'm tempted to agree with mamadoc that you should let sleeping dogs lie in respect of your dsis's h, I suspect the plan is to return him as soon as she's at home, or shortly thereafter, and as it's not unkown for such returns to made without prior notification he could fetch up on her doorstep at any time.
I therefore suggest that you make some discreet enquiry via your dsis's hospital social worker as to his welfare and reiterate that his dw is unable to care for him, and vice versa, should there be any intention of returning him to his home.
I also suggest that you cc any letters you write to consultants, social work/services etc, to all of the other individuals/bodies involved in your dsis's care and also to your dsis's and your own MPs.