I worry about my 86 y/o mom, too. She never got her missing front teeth fixed. She lost them while biting into broccoli. My sister at the time was caring for her, and my mother felt emotionally abused by her. My sister spent a lot of money on her own beauty treatments and teeth, but she neglected to care for my mom properly. I reported my sister anonymously.
My mom would send me messages back in 2017 and 2018 that my sister was causing her stress and making her feel like she would have another heart attack. I worried so much for my mom back then, and still do. Thankfully, my mom moved out and asked her granddaughter, my niece, to be her caregiver. At the time, it seemed like that was a great plan.
Flash forward to now and my niece (my mother's granddaughter/caregiver) remains unvaccinated with her two children she gave birth to during the pandemic. She's not married, and the man she's with isn't vaccinated either. It's horrible. There's another granddaughter (my niece) living there as well, and she is vaccinated. My mother is vaccinated, but she never got her boosters. I think she's struggling to find a ride. Meanwhile, my sister (the one who abused and neglected my mom) decided during this pandemic to move in with them. It is unclear if she is fully vaccinated or not. And just last week, they decided to go clubbing while leaving the great grandchildren (two of them) of my mother alone with her. My mother should NOT be having to babysit any children alone - not in her condition, not at her age, and especially given the fact that they are too young for any vaccine. That's not only neglect of both an elder and two children, but also child endangerment, elder endangerment, and exploitation for free childcare from a disabled/retired elderly person who is 86 years old!
But in all honesty, the other niece (my mom's granddaughter) does care for my mom, and she is vaccinated, so it's still better care than if my mom were in a nursing home (it's much worse care with unvaccinated and underpaid caregivers who don't socially distance off the job and who don't always wear masks properly on the job). So now it is the best of two evils, with hopefully at least one vaccinated family member who would report or stand up for her own grandmother if she sees abuse. It's just a full house with a lot of selfish people.
So I can totally understand your struggles and pain.
The difference is my mom has a ton of people in her life, and I'm estranged from them - save my occasional phone calls with my mother. So there's little I can do other than report, but I can't do that now during a pandemic. I wouldn't.
As for your parents, however, they might have alternative options if they are high-functioning and own or rent the home they live in now. A social worker or other authority could kick your leeching brother out and provide your parents with at least a caregiver who could check up on them every day or every few days. If they are in need of more care than that, there might be other options as well, to avoid nursing homes.
You could also have an attorney assist with agreements to protect your parents from signing over any Living Will agreements to your brother. Sometimes Victim's Advocates can assist with that.