You could pay for ALOT of caregivers with the $400,000 (at minimum) that the sister saved by living rent free for 35 years. It has been my experience that caregivers and end of life care are very expensive, but they aren’t needed for that long. I still think the sister who has lived rent free for so long has had a pretty sweet deal.
OP states she paid rent, but I'm sure not going rate since all she likely had was a bed & bath, but who says parents had the cash to pay for caregivers, especially when it came to 24/7. And daughter also gave up privacy, maybe even the opportunity to get married and have a family ...
What Mom and Dad did regarding their children living with them while they were alive was THEIR business and has nothing to do with the estate, unless will included in writing some stipulations. In many areas of this country and the world there are multigenerational houses where the parents might own the home but other family live there. I don't think because it isn't normal to someone that makes it abnormal with consequences. No reason to look down on folks for multigenerational living.
I have two adult sons who live with me. DS#1 is permanently disabled. I am his primary caregiver. When we pass it doesn't mean that he will get less of our estate. DS#2 rents a basement apartment from us. While we appreciate the rent money what I appreciate more is the assistance with physical or difficult things, the backup he can provide with his brother, dog sitter when we are gone etc. As a teacher and single, current rents are quite high for him alone so it is a win/win for both of us - and he can save money towards buying a small home. Will he get less than DD who is married and hasn't lived here in years? NO. He helps in ways that help us and save us money (like OP's friend).
In the end the ONLY thing that matters in this situation is what was happening in the last year since Mom died because it is no longer Mom's decision. This is now an estate situation. Executor can't go backwards before Mom passed, it is not her concern. But there should have been clear discussions on what happens now - a year ago right after Mom died.
Some wills will specify that the executor will serve without compensation. In that case a person named as executor (or alternate executor etc.) could decline being or becoming executor if s/he wishes. The estate has the option of include that person among choices for an alternate alternate executor or for a yet alternate executor under different terms including compensation.
The heirs or beneficiaries can also "fire" an executor and choose a replacement.
Three down, and NONE included compensation. I think this often happens when the Executor is family and also one of the beneficiaries.
Yes to declining the job, then it goes to the next person in the succession. AND yes there could be a request for a change. Some wills are decades old and sometimes the person who is Executor is not longer in a place to handle the work load but doesn't want to not fulfill. Then the problems start.
That happened with us. CO-Executors, neither wanted to do it, both ill equipped to do it AND neither had any idea of her financial situation. BUT they said since they told her decades ago they would do it they had to stay Executors.

Since I was the only one who knew all her financials and had access to her safety deposit box, bank accounts etc I ended up doing most the work, getting them to sign stuff etc.
FOLKS need to look at their wills every ten years to see if anything needs updating !!!!!