What is the ruling on my sister making up the days she missed in Ramadan due to pregnancy, breastfeeding, and illness, and does she owe expiation?

The sister is obligated to make up for the missed Ramadan fast. If the delay in making up the fast until the next Ramadan was due to a valid excuse, then she is only required to make it up. However, if it was without a valid excuse, then she must repent to Allah Almighty and pay an expiation for each day she delayed making up. She is not required to make up the days consecutively; rather, she may make them up intermittently.