I have never asked my mother-in-law about her diet.
What do piranhas eat?
In the wild, a piranha diet consists of insects, fish, crustaceans, worms, carrion, seeds and other plant material. Some piranhas do occasionally eat mammals, but usually when the animal is already dead or gravely injured. Attacks happen, but they aren't as common as the movies would like you to believe.
To feed an aquarium piranha, there's lots of advice here.