This is a classic example of MNers making themselves appear like Mother Teresa on behalf of some unseen character in the drama, whilst actually using the occasion to be rude and dismissive to the person asking for advice
^ This.
Op I really feel for you, dementia is hard on everyone.
I think your mistake here has been to toil away selflessy for years and not had this conversation before.
I understand why you want what you want, I think you would have saved them a fortune in fees etc, and also surely peace of mind?
I do not trust care homes at all, and I would not want a relative in one especially not with dementia.
IF i was the other brother, I would want you to have a bit extra for having the strength and skills to care for my mother!
Op, life isn't fair.
I know of a couple who cared for elderly aunt, no dementia but physically disabled, they cared for her for four years, and they knew she nad her children had no home. she left millions to cats, and a very small lump sum to her own family.
In your situation I would simply accept the will/money or fact brother is getting half, and pull out, and start your career again.
You cant build your life on sand. You are the fool for giving up so much without concrete assurances first. on the other hand, perhaps you didnt realise your quality of life would dimish.
good luck.