You have to have 2 independent medical assessments at a minimum as well as written consent that is witnessed. So its not like you can just say you want to do it and then they just off you right there. She could have had all sorts of reasons for not telling anyone including her sisters. There's nothing in your anecdote that disputes she could have planned it long in advance and just not told anyone.
It seems implausible.
She lived with her sisters and while she was quite capable of many tasks, I think that long term subterfuge was beyond her. She was well into mental decline.
"You have to have 2 independent medical assessments at a minimum as well as written consent that is witnessed"
Could this not have happened at the hospital?
Yes, it could have happened at the hospital but do you think they have people sitting around waiting to do medical assessments and be witnesses just so they can push MAID onto people? At most hospitals the doctors and medical staff are extremely busy.
Which is ironically a pretty strong argument for why euthanasia shouldn't be legal