Your sister could well have ME (Myalgic Encephalomyelitis). These words mean "muscle pain" and "brain inflammation". Personally I am in a remission phase, I caught the disease in 1995 and it took me about 3 years to sort-of recover. My blood tests were all normal.
I read an article in Reader's Digest about ME and it didn't tie up. Obviously whoever wrote the article had no first-hand experience of ME. The trouble is, most people with ME are not up to writing well about it. A few notables do. An affected doctor, Charles Shepherd, has written much sensible stuff.
I have a personal analogy, which explains all symptoms. Imagine your brain as a pinball machine. The ball (virus) bounces about, hitting one area then another, virtually at random. Your brain has a vomitting centre and it can hit that. OK that's simplistic, but it DOES affect all the brain and differently in different people.
The disease ME used to occur in outbreaks in isolated communities, and was given various names, like Icelandic Disease, Tapanui Flu, and notably Royal Free Disease. It attacked most of the staff at the Royal Free Hospital in 1955 or the early 1950's (sorry I can't remember the exact dates offhand but you can find them easily). It attacked mostly the working population, not the patients, a unique feature of the disease. Around 256 doctors and nurses were hit - some recovered, some had long-term effects, just as nowadays.
Unfortunately, because no-one could find the virus that causes it, the Psychiatrists got hold of it and said it was a "mass hysteria".
Previously the Psychs called MS (muscular sclerosis) "a hysterical disease" - they would (and will) be out of business when we find the root cause of ME. The disease is much more "endemic" now, with easy air travel, so clear-cut epidemics are rare.
The Psychs have tried to change the name to "CFS" - Chronic fatigue syndrome - and thereby hogged the subject for themselves, claiming it is "all in the mind", and gave it the trivial title "yuppie flu" - meaning it affects only working people, and is caused by their deranged thinking. This suits the insurance companies and the Governments (of all countries) who would otherwise have to pay out for sickness benefits for physical diseases.
ME however occurs worldwide, regardless of culture. If it were purely psychological, wouldn't it be different between different cultures?
There is now enough evidence of physical changes to the body and brain to define and diagnose ME (see http://www.youtube.com/watch?v=hkGq0BH6AHw
There is little if any REAL research being carried out into the actual cause of the disease, as all governments have been willingly hoodwinked by the pyschs into calling it a psychological disease. The MRC (Medical Research Council) in UK is responsible for government financing of all research projects and is still funding "research" into cognitive behaviour therapy (teaching you that it's all in your mind) and graded exercise therapy (gradually increasing your exercise rate). There is an alarming number of alternative therapies being promoted, from simple vitamin treatments to "Lightning Therapy" - well Esther Rantzen's daughter was said to have recovered using this form of CBT (cognitive behaviour therapy). It's a "Talk Therapy" - 'nuff said?
If it were that simple, we'd all get better. Most sufferers have tried everything, despite their poor financial positions. The quacks are preying on them. It doesn't work.