Massive meltdown accusing me of trying to ‘get rid of her’ and ‘you’re trying to kill me’. Wild mood swings.

before you do or say anything explain exactly what you are doing and why in simple terms. Say it all in your father's presence. Directly ask his opinion and ask why he thinks and does that.

They’re going to see this as very spiteful,

this is just sad, I'm so sorry

You could challenge them publicly, ask them to write a list of all the "horrible" things you have done, because they can't

you've got to get your sibs in on this

even if they do nothing the least they can do is back you up

if she wasn't old I would say it's anorexia/eating disorder and an act of defiance and trying to control her environment and other people