grandma? why is she so young?
Did you just skip the whole thing, lol.
They're actually twins. They come from a family of priests that drank a special tree sap as they hit adulthood, one granting health and longevity (probably even immortality). Fuuka, the younger twin, has born sickly from the start and was dying when they turned 18. So grandma abandoned her family tradition, fed that sap to her sister and ran away with her. Now grandma gets to have a sister and live out a regular lifespan, while her sister gets to live in general and be forever young.