Im very sorry to hear about your sister. Now there can be a number of conditions which are causing her unconscious state, i assume that she would be in the ICU right now. First ill let you how brain stem death (the medical term for brain death) is diagnosed. In order for a patient to be labelled as brain dead, she has to have the following signs,
1) Fixed and dilated pupils which are unresponsive to light
2) Absent corneal reflexes blilaterally
3) Absent Gag reflex
4) Absent Dolls eye reflex
5) Absent vestibulocochlear reflex
Now all of these tests are done clinically at the patients bedside, if they are absent a suspicion of brain stem death is made and then the person is observed for another 24 hours during which time two more examinations are done. if they all show these signs then the patient would be declared brain dead. Secondly, in most cases confirmatory tests are also done including a EEG which would show no brain activity, a functional MRI scan or even cerberal angiography in some centers. But in most cases only a EEG would suffice as that would show the brain activity.
The above clinical signs are the only signs of brain death and there are no other symptoms per se. A person can have normal heart beat, normal blood pressure with functional kidneys even if the brain is dead. However all of these parameters would start falling soon and would require artifical support.
Now in your sisters case, given her stage 4 cancer, it is very likely that the cancer has spread to the brain and is now causing and increased amount of pressure in the brain. this would cause her to fall unconscious. It is also possible that she might have bled in the brain due to the cancer spreading to it. Both of these conditions would be diagnosed with a CT scan of the brain or a MRI if needed. In both cases not much can be done, if cancer is the cause. Other causes for her unconscious status can include severe electrolyte imbalances or even severe infection in the body which can occur because of her decreased immunity.
You havent mentioned if she is on any support which means is she on a ventilator? Is she being given any drugs to support her blood pressure? If so that is not a good sign.
Do you have any more questions?
Please leave a POSITIVE rating by clicking on the stars above, until you do i am not compensated for my time. You can continue to ask follow-up questions even after the POSITIVE rating. You can also ask questions from me directly by going on the link below,