First of all George Lucas based part of Star Wars on Greek mythology. One of the stories in Greek mythology is that of Hercules. Hercules' had a twin brother, BUT they had different fathers. Hercule's father was Zeus while his brother's father was their mother's husband. You with me? So here's my theory: Leia could possibly be OBI-WAN'S daughter. Here are my reasons... When Anakin turns to the dark side, Padme will want to find comfort in someone else. Why not let it be the one who first came to be her bodyguard? The one who would be closest to Anakin and therefore, closest to Padme? Surely Obi-Wan would be the first to find out about their marriage. He'd have to keep it secret too. And why is it that Obi-Wan must watch over Luke? Did Padme ask him to? Maybe to keep him away from the daughter he'll never know? And Vader doesn't act to suprised when he finds out he has a daughter. He says "A sister!... Maybe she'll join with me!" (Or something like that.) He never says that she should join him BECAUSE he's her father. I think that somewhere in there Padme had an affair with Obi-Wan and Leia is truly Obi-Wan's daughter.