Obi-Wan: 'The emperor knew as i did if anakin, were to have any off spring, they would be a threat to him'
Luke: Yoda Spoke of Another?
Obi-wan: The other he spoke of was you're twin sister
Luke: but I have no sister
Obi-wan: To protect you both from the emperor you were hidden from your father when you were born, and that is why you sister still safely remains annoymous