I will try to address these in order... 'twas a long night last night, so I am functioning in slow mode.
Her reaction was comical... she told me I don't spend enough time doing nothing as if I would be doing nothing during the time DD was not with her nurse . My house was too clean (bollocks!) and DD gets too much attention (cognitive impairment, physical disability and medical needs tend to do that). I have my MN time, and some quiet time once DD is in bed. I am not a person who likes doing nothing - it is not my style.
I don't live in the UK, so things like direct payments are not an option. Currently, the "respite" program we have is subject to intense debate... as all of the families that have been approved in the last 8 years, instead of receiving a funding allotment, are then put on a wait list. So, while I know how much we should receive for her respite care (the highest funding level), we receive none of it. She does get a private nurse at school, and through medical home care, we get one overnight nursing shift per week (so I can get some rest) but that was cut back from 7 nights a week. While being "on call" to respond to alarms etc all night 6 days a week is definitely taxing, having the house occupied only by family most nights is enjoyable. Try to imagine having essential strangers in your home while you are sleeping every night. You can imagine what that did to the intimate side of my marriage
Like I said, I enjoy doing little things for DH - and making his lunch or doing his laundry is hardly a big deal. Frankly, as I do the shopping, it is easier if I make lunches too - that way we don't run out.
DS does watch his sister, but he needs to have his own life, too. With his sister being half his age, and her needs having seriously impacted on his life already, we need to be cautious. He loves his sister immensely, but it would not take much for him to resent her terribly, as well. He will be watching her today while DH and I go out to look at cars.
Right now, I am admittedly treading VERY carefully with demands on DS, as I feel terrible for something that has happened with him. I was supposed to be taking him on a trip for his 16th birthday - we should have left a couple of days ago - but the care I lined up for DD decided her care needs were too great and backed out too late for me to find alternate arrangements. DH could not take the time off work - he is new in the position. This was supposed to be a once in a lifetime trip and had to be cancelled. He is justifiably upset, and there is not a damn thing I can do about it. Once again, his life gets turned upside down through no fault of his own.
I have ways to amuse myself, but they are home based. DD can end up in the hospital 200km from home on a moment's notice, so committing to something financially that I attend has proven to be wasteful.
jimjams understands the challenges of "respite" very well. We trust this nurse implicitly, and her daughter and DD are very close. The nurse takes DD out into the community and (more importantly) stays familiar with changes in her condition so that she will be up to speed when school reconvenes, as DD's condition is degenerative and very variable.
As far as me cleaning when my friend "pops in", perhaps if she called first, I might actually be able to give her 100% attention. As it is, I finish whatever I am doing, put the kettle on, and take a break. Perhaps not good enough for her 
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I do my own cleaning because a) DD has an inborn immune deficiency, so once/twice a week by a cleaner wouldn't be sufficient; b) I actually like knowing I am keeping the house myself, especially since it was something I struggled terribly with in the past; and c) when it is broken down, it is only the regular daily stuff plus one or two extra jobs every day anyhow.
But this morning, we are having a quiet time. DD is watching her favourite video (today) of polka dancing and playing with the dog and I am MNetting. Night nurse bailed out last night at short notice, so we'll be going a 13 day stretch without night care. Par for the course...