Ludoole, that's a lot for one person to deal with - no wonder you're feeling a bit lost.
I know something of what you're going through. My parents are also in their 80s. Dad has dementia, along with various other health problems, and Mum has always been prone to depression, which got worse as his dementia worsened. Their marriage was not a happy one to begin with and Mum didn't have the patience to deal with his dementia. For several years I cared for them both on a part time basis, along with help from some paid carers, but in the end the situation became untenable for everyone involved, particularly Dad. Last year he went into a care home, and is now happily settled there. Mum is still living in her own home, with paid carers to support her (she has no real health issues but self neglects and needs a lot of prompting on hygiene matters).
I am lucky in that I have a very supportive husband, but unfortunately he now has health problems of his own. My husband is also my best friend and I have no close friends to lean on in RL. In your position, I would spend as much time with your husband as possible. You mention that your brother is unhelpful/unreliable. I think maybe you need to be a bit blunt with him and explain that he needs to step up and take his share of responsibility in supporting your parents in their old age. If he really is unwilling/unable to help then contact social services and ask them for a full assessment of your father's needs. SS may be able to offer a support package of carer visits, to take the pressure off you all. As the disease progresses, you and your mother are going to need a lot of help so it is wise to get as much support as possible from any source available. It may well be the case that your father's needs grow to the point where he needs residential care, but in the meantime you need help now, so please seek it.
Sadly, your father is very likely to live months or years longer than your husband, so in your position I would do whatever you need to do to make the most of the time you have left with your husband. Your parents' situation is very sad, but bear in mind they have had long lives, so when weighing up their needs against your husband's needs, it seems reasonable to put your husband first, which may involve stepping back from your parents' situation a little, because you can't go on as you are.
I'm so sorry you're going through this. Life is just sh** sometimes.