If the reality is as you have mentioned, your sister is not to blame if
she recites Al-Fatihah only and forgets to recite the other invocations of Salah. She does what is within her capacity and is not to be blamed for what is beyond her capacity to do or memorize. She is excused for her mentioned illness, as Allah (Exalted be He) says,
Allâh burdens not a person beyond his scope.
Allah also says,
So keep your duty to Allâh and fear Him as much as you can
However, you should strive to teach her the first and second Tashahhud and the formulae prescribed for Ruku‘ (bowing) and Sujud (prostration).May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.