What is the ruling regarding my grandmother's prayer, as she is over ninety and prays sitting, and forgets Surat Al-Fatiha and the number of rak'ahs? Is her prayer acceptable despite her many mistakes? And what can we do about that?

Allah the Almighty is merciful to His servants. Therefore, whoever is unable to fulfill a condition or pillar of prayer is absolved from what they are incapable of. What your grandmother is unable to do is waived for her, and her prayer is valid. Strive to remind and teach her. If she seeks assistance in counting the rak'ahs by bending her fingers or similar methods, there is no harm, as Sheikh Ibn Uthaymeen mentioned. It would also be good if your mother or someone else prays with her, because, according to the majority of scholars, the ma'mum (follower in prayer) is not obligated to recite. So, let this woman do what she is capable of, and what she is unable to do is waived for her. Her remembering some worldly matters does not negate her inability to perform some actions or recitations of prayer.