The secret of Luffy's sister

Luffy shows up after two years with a young woman. Her name is Monkey D Luna, big sister of Luffy and a pirate. The straw hats are surprised because they did not know about a sister before. Why does she show up now? Where has she been all this time? How comes Luffy never mentioned her? What's her purpose? What crew is she with?

[A translation of my story "Das Geheimnis um Ruffy's Schwester]