So, Ginger is Luffy's biological big sister. So, why is it that Dadan never saw her? Why did Ace never mention having a sister? Why didn't Luffy mention a sister? Why didn't Sabo's memories come back upon seeing Ginger's face?

Well, since Dragon took off not long after Luffy was born, Ginger was left to wonder why her father left her behind. While the people in Foosha Village told Ginger that Dragon abandoned her and Luffy, she refused to believe her Dad didn't love her. At eight years old, Ginger ran away from Foosha to find her father.

Since Luffy was only 5 when Ginger ran away, he doesn't really have any memories of his big sister. And since Ginger wasn't there when Garp took Luffy to Dadan, Ace and Sabo never met her.

Ginger spent nine years searching for her father. At seventeen, she actually encounter Ace and Aoi, who over heard the name "Monkey D" and thought she was looking for Luffy.

Aoi, who actually has a friend in the revolutionary army, called up her friend and got Ginger to the base.

From then on (after extensively arguing with Dragon about it) she was the weapons specialist in the Revolutionary Army.

She didn't know Sabo, and Sabo didn't know her.

Though Ginger did try to get to the War at Marineford to save Ace, but Dragon refused to let his Daughter put herself in a war with marines.

That is how Sabo learned Ginger's true identity.