My sister was pregnant in the Ramadan before the last one. She could not fast back then. She gave birth, praise be to Allah. She could still not fast thereafter because she was breastfeeding the child and was sick. The following Ramadan came, and she was okay. Thanks to Allah, she was able to fast. My question is about the Ramadan that she missed and that she was not able to make up for. Should she start making up the missed days consecutively or separately? Should she pay an expiation?

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.