That is your right. I would not want to live like a vegetable myself. But, I would make dang sure to have it in writing.
All we have are the husband's claims along with his brother and brother's wife.
Even if Terri did say she wouldn't want to live like this (which I don't believe she did in her early twenties) people do change their minds once something like this happens to them. There is usually a strong will to live no matter their condition.
I would guess that 9 out of 10 people "would not like to live like a vegetable." That leads me to logically infer there is 90% chance Terry's wishes are being carried out.