Hello & Welcome to Just Answers.
This is a tough situation, but since you're in the field, I'm going to tell you what I really think and hope that you can hear that I have compassion for your situation.
When you have a family this dysfunctional, you can't hope to have a normal parental illness or death. The crazy that's already there spirals out of control and the sane person gets trampled on as they try to 'do the right thing' and get proper closure and help out the elderly relative. It just doesn't work if you have any expectation of normal with the other relatives. Adult children of the dysfunctional can't do the 'normal' things when their parents become infirm...it just ends in utter chaos, and the unreasonable relatives will blame all of it on you. They'll project it all on you and you'll be the scapegoat no matter what you do.
In light of that, I suggest to people in your situation to be very careful of 'romantic' notions that when things really turn for the worst health-wise with the elderly parents, that you back off and decide exactly what it is you want to achieve for yourself only. Make your expectations low, get in and have a talk or say what you need to say and then back out as far as you can go.
The dysfunction in families HATE the ones who get away. It's just too scary for them. They don't come around and they don't change. Let it go. Mourn your losses and stay far away. Imagine them to be in an aquarium like interesting fishes, but nothing more intimate, personal or deeper than that. They're not going to let you help.
Its more likely that they're going to abuse your mother like she probably did
them, and you won't be able to stop it. Everyone has their own way of working through childhood pain, and for some people this happens when the parent becomes infirm. They really take it back out on them. It's not something you hear spoken of much, but I see it often as abusive, unprotective or personality disordered parents become incapacitated. Payback time is a difficult thing to watch as the wolverines circle the infirm parent, but there isn't much you can do about it without the pack turning all their rage onto you.
I suggest you step back and take care of yourself while mourning your losses.
My best to you.
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