I have experience of a controlling sibling. A brother has a terminal illness. One sibling took 'control' of the situation and this is what we had been/are seeing - the gatekeeper sibling controls all contact with health professionals. She literally rang them all without conferring with rest of the family, and told them she was now in charge and was the main contact. She limits access to the sick sibling, emails other siblings telling them they don't care enough, are bad people and don't love anyone, are bullying the sick person, are doing things wrong etc, are phoning too much/not phoning enough. She also told rang and texted and emailed everyone telling us the sick sibling only has days to live, literally about 10 times in the last year. Based on her own opinion not the doctors opinion.
Takes on far too much responsibility and then blames others for 'making' her do it. I could go on and on.
In my case the controlling sibling was virtually bullying the sick sibling as well when they didn't behave the way she wanted. No one can say a word, because the controlling sibling is so sensitive that if you try to defend yourself, she will then accuse you of bullying her or interfering with the sick siblings care. Also if anyone questions her, the same thing, you are called 'aggressive'. She has also cried in front of the sick sibling saying other people were being mean to her - and this is a grown woman! So we have to let her away with everything she says to us, or my sick sibling will end up feeling bad over it. It's pretty tough.
She also manipulated the sick sibling into only letting her be the main contact at the hospice. So no one else was allowed to ring up and find out how they were. All our information had to come from the controlling sibling. We weren't allowed to come to the hospice without her permission, and she checked the sick sibling's phone to see who was ringing. We had to wait in reception of the hospice for her to come and get us and escort us down to my other sibling's room.
If you brought the wrong drink or chocolate for the sick sibling, or even sat in the wrong place at the visit, this was called 'bullying' as you had done it on purpose to upset the patient!
We are still in that situation to some extent, except that my sibling has been sick for longer than expected so the controlling sibling went off in a big huff, so another couple of us could get access and contact at that point. We are actually allowed to come to the hospice and go down to my sibling's room. Only because she won't speak to half of us now and so doesn't want to meet us in there. It's very tough!