Will an illiterate person who is unable to read the Noble Qur’an be held accountable for not reading it?

Your mother bears no sin for her inability to recite the Holy Quran, as it is not an obligatory duty. She can compensate for this by listening to it, because Allah Almighty says: (And when the Quran is recited, listen to it and pay attention that you may receive mercy.) However, she must memorize Al-Fatiha because prayer is not valid without it.