I think you maybe need to start enforcing boundaries wit your brother too - hang up if he rants, say you're not talking about it any more unless he sits down with you both together and agrees a plan. Agree with your sister in advance what the line is and stick to it.
I think the sibling thing can be hard to negotiate - my sister who died got to the point that she realised how bad my Mum was ahead of the rest of us, but we were both a lot younger and didn't realise, which I feel really bad about now.
I ended up with counselling and my sister still talks to my Mum more than me but is getting increasingly sick of her.
DF visit was crap as usual. DF slept through it, although his eyes opened briefly I'm fairly sure he didn't know we were there, maybe that someone was.
I ended up getting DH to let my mum know we were coming. She replied back saying that DF had had a lovely day trip and was very alert yesterday, and that would know we were there and understand what we were saying.
She quite frequently pretends he has conversations with her when no-one else is there so I guess he can fill her in on everything we said. 🤔The fact that he's comatose when anyone else sees him is always because he's had a trip out or a busy day before or some bollocks like that.
The pretending he's less sick than he actually is is part of her justification for trying to get every medical intervention under the sun, part emotional blackmail to try and get us to visit more, and also - and I can see this one coming a mile off - will be used to justify all sorts of terrible behaviour when he does die, because it will be "a shock".
I know it will be hard for her when he does die, but the way she campaigns to keep him alive in such a terrible way is horrible.