Accidental mushroom poisoning isn’t terribly rare among amateur foragers who have no idea what they’re doing. So her saying that she just bought them off of someone is somewhat believable.
The unbelievable part from the beginning is that all the people that ate the deadly mushrooms were her ex-inlaws while AT THE SAME MEAL she served her children and herself a completely different set of food. She cooked two entirely different meals. Magically she and her children didn’t eat the deadly one.