Hello,
Stomach flu doesn't normally pass on from one person to another in groups so quickly - sure sounds like food poisoning to me. Just had myself. No actual being sick, retching all of the time, very upset tummy, fever, talking gobbledygook - you name it I did it. Mine lasted 2 1/2 weeks - mind you I have got other medical problems so this is what took it so long to get out of my system.
I do hope your parents get well really soon.
Take care
goldenwings