Last year ND had 22,200 applicants for the class of 2023 and accepted 3,410. That is roughly a 15% admittance rate. I haven't researched the numbers on ACT/SAT scores, but I should think your niece's test scores are in the competitive range. 47% of the students accepted in the class of 2023 attended public school, 34% Catholic school, and 19% attended private/charter school.
Because so many kids applying to ND are high academic achievers in high school, Notre Dame has applied what's been described as a "holistic" approach to admissions: "Notre Dame has a
holistic admissions process involving other factors beyond your grades and test scores. A
strong application essay and
glowing letters of recommendation can strengthen your application, as can participation in
meaningful extracurricular activities and a
rigorous course schedule which includes AP, IB, and Honors coursework."
https://www.thoughtco.com/notre-dame-gpa-sat-act-data-786727
I've personally been told by Don Bishop, the AVP in charge of undergraduate admissions, that extracurricular activities are an important factor that admissions will consider. So tell your niece it's important not just to bury her nose in her books in her last year of high school.
I was graduated from ND 40+ years ago. I can tell you that the admissions process is
way more competitive now.
I wish your niece success on her quest. Notre Dame is a special place. I look back now and realize my spiritual growth as a student there was just as important, and maybe more so, than any academic achievements I attained.