Hello Helene, we are sorry to hear about your niece's diagnosis.
Approximately 5% to 10% of people living with ALS in the U.S. have family members who have also been diagnosed with the disease, making it probable that a genetic mutation has been inherited. Sporadic ALS is the most common form of the disease, affecting approximately 90% to 95% of people living with ALS.
You can learn more about the disease here: https://www.als.org/understanding-als and have your niece reach out to a local care team at https://www.als.org/local-care.