Claims to have it, doesn't mean that she does have it - it doesn't mean that she's making it up either, sadly for many of us it can be a lazy arsed doctor misdiagnosis, or "waste basket diagnosis"
This can often lead to mistrust of the medical profession & reluctance to go to see a GP with anything you can diagnose manage yourself, why would you want to go & see a doctor who has already made you feel small for pushing to get a diagnosis & regain the life that has been ripped out from under you
All of what you describe with your MIL is common with M.E. - if your battery is flat, it's flat, there is sod all you can do about it but save those spoons & recharge that battery for another time. - Google "the spoon theory" for a better understanding of that.
That said, I agree with the others that think smoking & drinking sounds odd, it's more usual for sufferers to be intolerant of alcohol, plus you wake with a hangover every day anyway, so why risk a worse one by drinking.
It may well be that she actually has something else that the doctor has missed & they've made her feel like a hypochondriac for chasing a true diagnosis, so she's given up, she may have been made to feel that she is depressed or suffering a mental disorder & the more she protests, the more she convinces the doctor that she has MH problems, or she may just trust doctors to get it right, so accepts M.E. as a given.
A whole list of scenarios, none of which are uncommon, but lead your MIL & others like her to where she is now.
If you want to help her, find out if she has been checked for pernicious anaemia, see the chart I add below - new 2014 guidelines suggest that tests are unreliable & patients scoring highly should be given B12 treatment if they score highly with symptoms - I'm reading that many M.E. Suffered respond well to B12/pernicious anaemia treatment.
Also look into POTs - POTSUK is a good website for info - print off the "Active Stand test" that you will find on there & insist her surgery do it, make sure they do it properly though - I'm reading that up to 30% of M.E./CFS patients actually have POTs (postural orthostatic tachycardia) - (POTs is a symptom of B12 deficiency too) - if my surgery are anything to go by, I suspect that could be higher as it took them 3 goes to get the test right.
Also look into EHLers Danlos Syndrome - POTs is a symptom of this too
My Fibromyalgia & M.E. Diagnosis was wrong - it's taken 12 years & my daughter diagnosis for them to realise that I actually have Ehlers Danlos & POts as a symptom of that - POts can be helped with medication & it can make a big difference. They now realise I also have pernicious anaemia, which is linked & rife in my family