This has to be a user error because the chances of all surface burner valves going bad at the exact same time are somewhere between zero and nada.
How old is your sister-in-law? I have lots of elderly customers who one day suddenly forget how to use an appliance they've used for years. I can personally attest that this same phenomenon afflicts middle-aged folks, too.
As Brother Reginald has pointed out:
Hmmm - very strange. She is only 50 and doesn't usually begin drinking until after 5:00
I'll have to call her and interrogate her some more. She's sharp and i doubt it is her - she has been using this cooktop everyday for a long time. I completley agree that all of them going at one time would be virtually impossible - hence my suggestion that it might be a protective measure. Since that isn't it, I may have to suggest she call a repairperson (horrors).