All perfect praise be to Allaah, the Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.
If your mother was ignorant about the obligation of making up for missed fasts, then she is not sinful for having delayed fasting those days beyond their due time. However, she would immediately have started fasting the days that she missed from the very moment at which she came to know of the ruling. If she delayed fasting those days after having known the ruling and until another Ramadan came, then she is required not only to fast the missed days but also to pay a minor expiation for each missed fast that she delayed from its specified time. The minor expiation is to feed a poor person for each day of fasting that she delayed without a valid excuse.
Allaah Knows best.