One night, seventeen years ago...
Garp had almost fallen asleep on the wooden table as infant Luffy dozed in his cot when a blinding streak of light shot across the sky and landed on the seashore nearby. Instantly awakened, Garp recalled his father's words on his deathbed.
Flashback
"Garp... remember..." the dying man wheezed from his bed as the younger Garp gripped his father's hand. "Our family oath... wait for her... the 'Eternal Queen'... when.. she is... reborn... you will know..."
Garp finally let his tears fall as his father breathed his last.
Without a moment to lose, Garp grabbed his navy coat and hurried to the shoreline where he last saw the flash of light.
"Finally..." a disembodied voice sounded from a bright light orb that hovered over the sea. "An inheritor of the 'Will of D'. I am the conscious born of the 'Water Star'. I have been waiting for this moment. This child..." a shimmer of silvery dust trailed over to Garp's hands and formed a female infant. "Raise her as your own, descendant of Selena's retainer. Once she is old enough, she will claim her rightful powers. Until then..." the voice died off and sank into the depths of the ocean.
"As you wish, Water Dragon Goddess." Garp muttered, tightening the cloth that wrapped around the baby girl and headed back to his cottage. "Well Luffy." he looked at his grandson. "Looks like you have a new sister. Now, what should I name you?" he turned to his new granddaughter, the girl's bright eyes gazing up at him. "How about... Sasha?"
The newly named Sasha babbled joyously.