Try an evaluation with a speech pathologist. Many specialize in cognitive re-training, including the various types of memory losses that can occur following a CVA. They also teach compensations to aid in dealing with everyday occurrences. Your sister should have a pretty good prognosis, considering her young age and time elapsed since the stroke. However, it also depends on the locationand severity of damage.
You can locate theses specialists in hospital outpatient clinics, private practice, or just go online. *edit* Good luck.