There is nothing in the OP's post which says she has made a slave of herself to help her parents and sister. She says she has offered some help to her parents only, but it turned out that the help they want is connected with the care of her sister, and that is something she doesn't want to do, and I don't have a problem with that. I responded to her post and am copying it here for you, because I initially posted in the light of my experiences.
I'm disabled. I have hidden - unseen - extras. People look at me and wonder why I use a wheelchair. Even some carers in the past have thought I could do more than I can. You actually cannot tell by looking at people how disabled they are, even if they wear splints. Sometimes you get afraid to smile and laugh with some people, because they think you can make a better effort.
I put up with it.
There is no way I would look completely dependent. I have done far too much to lose my independence to family and friends. I do think that your DPs should have help looking after your DS - but from outside the family and pay for it. Your DP should realise that. Perhaps they have made her dependent on them because they wanted to keep control?
I get benefit. If I pay someone, I call the shots. Perhaps your DPs are afraid of that? Perhaps DS would rather pay someone else? I certainly wouldn't want my siblings to take me over, nor would I ask the SS either. I pay my carers privately, and really, that is what should be happening with your DS and DPs.
Local social services are NOT free in most areas of UK. If they are now, they won't be for long. They use agencies which charge very highly, and employ carers on minimum wage. The charge on the council budget is the difference between what the agencies charge them and the wages of the carer, because that is passed on to the client. I meet my carer's half way, paying them just a bit less than the agencies charge me when I have to use them.
The SS provide help you have to pay for - by the quarter hour. How much do you think someone can do in that short time to help anyone? ALWAYS find someone privately - who is self employed, with qualifications in care, insurance, have their own car which is also covered. Also, make sure they have a visa/permit to work in UK. Don't be afraid to ask. You have to. The law expects you to. They also have to be registered with the police as far as committing crimes are concerned and for going into other people's homes.
You can get all that online once you have their name and details, and car registration number. They should give you all that so you can check. They can also print all that off themselves and give it to you, together with their driving licence to show their driving history. It takes minutes, so is no arduous task. If they do not give it, don't employ them.
There are facebook groups with ads for carers and cleaners. They will give you their details when you ask them to interview. They need work. You can place ads on these groups yourself if your family cannot. Just give your postcode or the town and you will get messages from those who are close enough.
If your DS doesn't receive UC and PIP she must apply for them. If your DPs are having physical and mental problems, they should apply for AA. All that can be applied to paying for outside independent help - privately. If you pay a bit more, they will be a lot kinder than those earning minimum wage and rushing about.
Your ds and dp will have to pay someone.
It is very important. I know if I am without help due to holidays etc. I go downhill quite fast. There are people who really love to look after others, and love their work. They would have no way of fulfilling their vocation if there was no one to look after. If they are appreciated, they will stay for years. Find someone with other work, perhaps courier, who meet other people.
Be flexible. Don't demand they come at the same time all the time unless they want to do that.
The only burden on you is to make sure they do all that and pay for it. You can always treat them to groceries or repairs if you think they may be short. It is surprising, though, what you can budget for once you realise that paying a carer privately is important.