my family can euthanize me even if I object?
No.
There's no law that allows killing of the unwilling; even a living will addressing assisted suicide or euthanization due to incapability assumes that you would still consent at the later date, but lack either physical or intellectual ability to communicate that. If you can clearly communicate that you've changed your mind, they have to respect that, even if that changed mind has reduced capability due to dementia.
Your best hope would be to go with assisted suicide while you still have enough faculties to make the decision and execute on your portions of the act.
I'm always so torn on him; he makes interesting videos about compelling topics that are reasonably well researched ... but as nitpicky as it is, I cannot deal with his accent for very long. I'm completely willing to believe it's actually genuine, but it sounds like an American doing a bad impression of a posh British lordling.