How do you deal with both parents developing dementia at the same time?

They live together. Have fought all their married lives. 65+ yrs. My mom was told that she has needed anxiety/depression meds her whole life. Two drs. told her she needed to stay on it for the rest of her life. She goes to her PCP provider alone. Tells him all her marital problems. I went last time because she claimed he didn’t do anything for her. The Dr. told us she was like a Grandma to him. She calls and gets what she wants from him. She needs therapy and/or meds to stabilize her. He said she doesn’t have to do anything. He doesn’t try to convince her to do anything, just protects her. Been going on for years. He’s like her therapist, confidant. She says she’s losing her memory and wanted to try Prevagen. I tried to get him to give her a memory test to see how or what kind of medicine might help with her memory while we were there. She absolutely refused.. She was verbally abusive to me and the med assistant in the office. A few months ago she called the Dr and told him she wasn’t taking Zoloft anymore because she has a rotten husband. We didn’t know. Just saw the behavioral changes. She definitely has symptoms of dementia. Dates are mixed up, suspicious of children stealing, argumentative, etc. She has always been a little off. I found yoghurt stored in the cabinet. Throws sanitary wipes in the dryer with clothes.
My dad has some major changes going on, too. Also suspicious, asked me to start helping him pay bills, doesn’t know what day it is sometimes. Imagines handwriting on bills.. Sits for hours staring out the window. These two are like fire and gasoline. The fighting during my childhood was so frequent and extreme that the police were called to break it up often. The arguing can escalate, and I see that it is instigated by both parties at different times since both are easily agitated now. They’re doing this in front of me again. We were trying to put together a grocery list. I walked out one time. Neither one gets around that well. My mom tries to cook and clean. Neither has any friends, and these last 2 years of seclusion have taken its toll on them.
There are 8 children. I’m the 2nd oldest, 1st daughter. I am 62. Been retired for almost 5 yrs. I have increasingly been taking on the role of caregiver over that time. I am ready to get back on the road, the reason I retired early. I’ve been taking them to appts and surgeries, getting meds, groceries, etc. I do not want to bear this alone. My siblings are in denial of their condition. My parents don’t want to ‘bother’ the others because they are working. They will not allow anyone to come and clean, help care for them or bring meals. They reject home cooking by others. They have the financial means for this. My dad is cheap. They don’t trust anyone. I am taking my dad to a neurologist this week. I took him two years ago after seeing some personality changes. He put on a good show. I am asking for a written test this time.
The worst part of all this is that there is so much uncertainty. My mom has a weak living will. My dad says he has one, but we doubt it. We think it’s a regular will, and he has hidden all of that from us, including my mom, for years. We don’t even know the attorney he used or if they are still in business. He keeps saying it’s not time yet. We are going to try to get a living will drawn up for him with HCPOA but hoping it’s not too late. My mom is in no condition to make decisions about his health.
I plan to cut back on my support. Calling a meeting with the siblings. Three of them live on the same street. Two others are also in town. I know you’re probably going to tell me to join a caregiver support group. I am not going to be the primary caregiver anymore. I should not shoulder that because I am older.I feel like I had to help raise my siblings, and now I have to be responsible, on the other end, for my parents. It’s time for the siblings to step up. They’re mowing their grass, shoveling snow, bringing their mail to them. That’s it.