Is it true that persons close to death (suffering from a terminal illness - eg: Brain Cancer) can "see things" and "know things" very much like a psychic and clairvoyant?

Not that I've personally experienced or ever heard about. Where did you hear this?

With the exception of a person at their exact time of death, no I have not seen this to be the case

My mother is terminally ill with brain cancer, and although she seems to be in a dreamworld most of the time due to her ailment, a few days ago she snapped out of it long enough to coherently give me a message, before returning to her trance.

I don't understand, based on your experience, what the question is? My late husband certainly hallucinated at times and also came out of his coma at times and was perfectly rational. However he didn't say or do anything that indicated any psychic ability.

Same with my Dad and Grandfather.