Alzheimer's/dementia

I have been reading through Gally's thread about being widowed and I wonder if any of you are suffering the 'living widowhood' of having a partner
who is suffering from some form of dementia. My daughter's much-loved MIL was diagnosed about a year ago, although we had all seen symptoms many years before. She has deteriorated rapidly and whilst she is still being cared for at home by her husband, who is a fit 72 year old, she now no longer does anything for herself and does not recognise her family. He copes as best he can, but he has told my SIL that he feels as though he is grieving but cannot move on as long as she is alive. They had plans to buy a boat and travel the world - he is a retired ship's captain. It may sound harsh, but it will be a relief when she dies - not just for him but for her, as she has no pleasure left in life and weeps all the time. She is only my age and has been such a wonderful step-grandmother and the best mother-in-law anyone could wish for.
I have told my daughter that I do not want to linger on if I am ever in a similar state and I am making a living will to that effect. However, I would not want my family to risk prosecution if they simply carried out my wishes
What a cruel disease this is, not only for the sufferers, but for their family too.