Steph
Diagnosis of narcolepsy is determined by a polysomnographic study which requires an overnight stay at a sleep disorder center. One variant of this study is the MSLT (multiple sleep latency test), which measures excessive daytime sleepiness. There is a difference between fatigue (feeling tired and sluggish) and sleepiness (capable of actually falling asleep) and a sleep study can distinguish between the two. You definitely need to convince your sister to talk to a neurologist that specializes in sleep disorders. Good luck.