The 62-year-old died on Sunday night after she was hospitalized just over a week ago, along with her sister, 58, who was in a less serious condition.
She died of thallium poisoning, likely caused by inhaling fumes from pigeon excrement over a sustained period of time, according to medical sources cited by news agency Ansa.
A police investigation found no traces of thallium in the woman's home, but noted that the barn of her country house - where she had recently been on holiday - was infested with pigeons.
Traces of thallium were also found in the blood of the woman's sister and the other family members who had joined them on the holiday, though it was only present in dangerous levels in the blood of the two sisters.
The woman lived in Brianza in Lombardy, and was treated at the nearby Desio hospital.