All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
Your sister has to fast the month of Ramadan that she missed. If she had a valid excuse to delay fasting until the following Ramadan came, then she is only required to make up for it. If she had no valid excuse for such delay, then she is required to turn unto Allah in repentance. In addition, she has to offer an expiation for each day of fasting that she delayed. She does not have to make up for those days consecutively. She may rather fast them over separate days.
Allaah knows best.