the key theme here is altered reality. or I should say altered perception.
you will see her wash her car and then she will forget she washed her car and try to convince you until sundown that she needs to wash her car and she will probably say that she doesn't know why she forgot to wash her car and that you need to stop stressing her out so she can remember to wash her car.
if you let it get to you, you will wear out fast and break down mentally and emotionally and just want to be done with all of it.
the way you win a negotiation is with irrefutable facts and evidence and data. no different than a court room where a witness conveniently forgets something to help the accused or vice versa.
keep records of daily activities. have her sign off on them
buy a voice recorder and have her dictate what needs to be done before she forgets it
keep all family "valuables" (photos, money blah blah blah) on a "loan out" basis to her and keep records like an accountant.
Plan all purchases and all sales beforehand.
these are obviously just starting points to convey a general theme
If you can't remember your life you can't control your activities or actions within it. Her life is going to get progressively more confused and divergent from your observed reality. That's why your family has to make every possible tangible effort to make her life conform to your reality as much as possible.
Remember two things. First, Her reality has already changed. The ball is now in your family's court. All of you will now have to adapt yourselves to her situation. Second, its a disease and at the end of the day at least its not cancer. If you don't develop a plan and a strategy for coping with all of this, it will surely wear all of you out and wear you down and overwhelm you. You have to accept the change, adapt your strategy, and address her day to day realities with a clear game plan in mind. If you don't do these things, the emotional and time and commitment strain on all of your lives will be too much and ultimately what you want is to have the best possible relationship that you can have with her given her cir***stances for as long as possible. Most people hate change and any kind of major life change like this can be especially wrenching. but what can you do? you can deny its existence and tune out, you can work within the system like Venus De Milo suggested by doing constructive things or you can just totally give up and either let her problems consume you or totally run away from them....but you will never ever be able to alter the fact that she has this disease now and its only going to get worse...so now how you and your family deal with it is entirely up to you. Her trajectory is already written. its your trajectory that's at issue in all of this.
Other than all of that...I am sorry this happened to you and is happening to you...you're a nice girl and life is hard enough in the modern world even without the added burdens of disease or dysfunction or whatever else. Most people are struggling just to pay rent out there. Try to keep your head up and stop and take a breath when it all seems too much. Anything can be managed if you just unlock the right strategy for doing so first.