It must be bleak out there with no replies. I know the feeling.
My advise would be DONT DO IT. I did this 2 years ago, the worst decision of my life. What was a lovely close relationship with a spinster aunt is now one of use and abuse - of me and 14 year old getting panic attacks due to stress!!! and yes lots of insults regarding expoitation from distant relatives who visit 2x a year and believe the ramblings of an alcoholic bitter old lady who is lonely and angry at her loss of youth/siblings and life. She has moments when she is absolutely evil to us-sniping, moaning, blaringly loud Radios and TV (not deaf at all!) rude to guests, messy eating, mess in room (hoarding mouldy fruit-they do do the strangest things - didnt have a clue of the extent of the idiosyncracies (sorry spelling)
with regard to the legality of the decision knowing someone has dementia (my aunt didnt/doesnt totally, mostly alcohol abuse probs) the one with dementia sadly died. Normally if someone is deemed by GP as having dementia back up diagnosis required by another doc - then someone has to have power of attorney or go to court of protection to get it. Depending on the closeness of your family relationship you may not get it. Other relatives have to be notified and can object - the court makes the decision. Court of Protection website - will send full guide. With regard to the sale of the joint property if they died or went into care- well care first - the council will take a charge or force sale of property (depending on what dependants you have living with you) to pay for care-THEY WILL NOT FINANCE CARE IF YOUR RELATIVE OWNS PROPERTY OR PART OF PROPERTY MORE THAN £20k they get their money eventually. If they die it depends on the terms of the last will written prior to dementia /or intestate?? if you have not been left their share of the property in the will you will have to sell to give the money to the beneficiary.
We had a deed of trust drawn up by 2 independant solc's one for each of us but a solcitor will not do this if they believe the relative has dementia (must be of sound mind). My cousin had to get consultants letters from hospital to solicitor to get her fathers will made leaving his estate equally to his children (wife dead) to prove in his early stage dementia he could still be able to make that decision.
Cant see how your siblings could have a claim assuming they are not your relatives children. Inheritance law goes down direct bloodlines or a will - so those who inherit are spouse-children-siblings-parents-cousins etc If they are your relatives children they all have an equal claim on the % share of the estate in your parents name even if they havent lifted a finger to help care.
Like a say a nightmare - think very very hard - get professional advise. Good Luck