Maybe a reason for more than one person in a family having fibro is what family members can share - dietary similarities, abuse, depression, learned behaviors like not coping with stress well etc?
Here's my family's example
Aunt #1 is a smoker, morbidly obese (5'3" 350 pounds if not more), didn't finish high school, does drugs, has 7 kids none of which live with her (the state took them away) Doesn't work, lives off my grandparents charity. Not to pass judgement, but #1 is kind of a loser. LOL
Aunt #2 is a Jazzercise instructor, eats healthy, has 3 teenage sons
Aunt #3 smokes, is relatively fit and healthy (not overweight but not buff like #2), 2 kids, very stressful job (attorney), divorced twice is single now
Those are the non-fibros.
The fibros are fraternal twins.
#4 is naturally thin-can eat whatever she wants and doesn't gain a pound, stay home mom of 1 daughter, married to a preacher
#5 is naturally thin, though not as thin as her twin, drinks those Shakeology drinks 24/7, works at WalMart as a cashier, has 3 kids
Then there's sister #6, my mother. She's a fibro. Doesn't work out but maintains a healthy weight, I'm her only child, is an administrator at a long term care facility (not too much stress), divorced, eats healthy (is vegan)
Besides having the same parents and the fact that they have all given birth to children, they all really have nothing in common. LOL They are all as different as can be. I have heard that the instances of fibro in women has been linked to childbirth. I'll have to hunt around for the article, I read it recently somewhere online.