Is it obligatory for a sister to make up the days of fasting that she missed in Ramadan 20 years ago, given that she paid money as expiation for them?

Your sister must repent to Allah Almighty for delaying the fast. She must make up the days she missed, and in addition to making them up, she is required to pay expiation (kaffarah), which is one Mudd of food for each day. The money given as a monetary value is not sufficient as a substitute for food, according to the preponderant opinion, due to the Qur’anic text: "And upon those who are able to fast, but with difficulty, a ransom of feeding a poor person." However, if this money was given to someone who provides food (like charitable organizations), then it is sufficient, and there is no need to pay the expiation again.