Reference: Silsilah Fataawa Noor ‘alad-Darb – Tape 2
Question: I have a grandmother who is more than 140 years of age, and is [only] able to move a short distance. [The issue is] she forgets some of the obligatory acts of the prayer, and every time I try and teach her the tashahhud and [soorah] al-Faatihah, and some of the short soorahs – as well as what is required to be recited in her prayer – she is unable to fully comprehend. And that is [because] after two days [or so], she forgets what I have told her. So, am I sinful [because of that], since I am [her] teacher?
Response: So long as you undertook that which was required of you in teaching [her], [then] you are not liable for any sin. So if you have taught her, and she [later] forgets, then you are not sinful [for that]. However, [continue to] teach her by repeating again and again. She is your mother or grandmother, so it is befitting that you take care of her, particularly since she has now reached such an old age. So she is in need of continuous [and repetitive] teaching, so she does not forget. As for that which you are [genuinely] unable to do in terms of teaching her, then Allaah has not burdened you with that, as He says: