Dagma, I am so sorry your sister has this disease and you are having difficulty accepting the reality of the situation. It is difficult to wrap your mind around a disease that has no cure or treatment. Yet I do not want you to waste the time you have left with your sister. As devastating as this disease can be you can find moments of joy and there are new memories that can be made. You don't want to miss these.
What you need to do is leave the past in the past. Your sister took over as matriarch of the family out of necessity. She stepped up when your Mom passed away, not because she wanted that roll, but because somebody needed to fill that roll. She took her turn and now it is your turn. It is not a roll reversal. It is just you stepping up as she did. Take it slow. A visit at a time. Leave the past behind and don't worry about the future. Just stay in the moment with her and cherish each smile you receive.
The "stubbornness" is a result of the disease and you have to say that to yourself over and over and over. It is not intentional or directed at you but a result of her frustration with her inabilities. A good piece of advice that all care givers learn is... DO NOT ARGUE! If she tells you the moon is blue then smile and agree how beautiful the blue moon is. As long as it is not life threatening let it go. Divert her attention (easy enough because of the short attention span) to something different and then go back to trying to do what needs to be done. Many times being positive and leading her gently will get her to follow. Also if you are at ease with a smile on your face, she will be more at ease as well. There are many techniques you will learn along the way if you just take the plunge and go be with her. Perhaps you should go be with her while her husband is there at first to ease your way into caring for her.
As for being paralyzed by this disease.... please don't let the disease win. It is not what any of us wish for our loved ones. I used to ask "why" my Dad and then my Mom had this disease and amazingly it was my Dad that gave me the answer. He just looked at me and said... "Why not?". It is what it is and it is there to be dealt with. Letting it paralyze us or denying it's existence is not productive. I have said this many times before but we are what we think. If we think we are overwhelmed and paralyzed... then we are. A simple change in your thinking will go a long way. Rather than waking up and dreading going to see your sister, make a plan to go and then assure yourself that you CAN do this. Please do not focus on the disease but focus on your sister whom you obviously love dearly. Do it for her
You can do anything for her.... and she has done for you in the past. Once you have been successful once, then you can do it again, and again.
Most importantly don't think of this time as "watching my sister die". That's a devastating thought. Think of this time as "helping my sister live as well as she can for as long as she can". Now there's a challenge with a positive spin. And that is what you want to do. You want to help her live. Fine something you and your sister can still do and enjoy each other and go for it. Amazingly you will find a new relationship with your sister that you will treasure. Being a care giver is a special bond that does have it's rewards. So just go for it....
Please keep typing. I do know how hard all of this is to do because I have been there and done it. It's all about how you think about it... so please be positive and hang around with us. We are here for encouragement any time you need it.
I might also suggest that you find a local Alzheimer's Support Group. It's a great way to learn tricks and tips for dealing with the disease and to connect with those that are dealing with just what you are dealing with.... and know we are here as well...