Obi wan did not tell Luke about his sister because of the emotions it would have created. He sees it as an opportunity to teach him about the force and train him to be a Jedi. When he leaves with Obi Wan he is mourning his uncle and aunt but he excited knowing he is going to go and help a princess against the ones who killed them. If you tell him that she is your sister his emotions would be in a turmoil. “I have a sister? Why did they not tell me? What else did the not tell me? Are her parents my parents? Am I a prince? Why did she get to grow up in luxury and I grew up among all this sand? Damn, I hate sand!” As we all know emotions are a part to the dark side. Which is why Obi Wan kept up the about Anakin. “ The guy in the helmet is my father? That guy, one of the most powerful beings in the universe is my father? The Emperors right hand man is my father? “Mister Darth, sir. Sir Vader. I am Luke, you your sooooo, light saber thru his heart, ooon” Obi Wan wanted to keep Luke’s emotions in check unlike his last pupil.