In ANH, I do think there is some sense to why Ben keeps it a secret that Vader is Luke's father. When Ben tells Luke that Vader killed his father, it seems pretty clear that Luke has no idea who Vader is or what horrible things Vader has done or even what the Force is. In regards to his father, Luke even says: "I wish I knew him..." Since Luke has no firm idea whether Vader is good, evil, or somewhere in between, Luke may have been tempted to connect with Vader on his own and maybe even ditch Obi-Wan altogether, and that probably would have led to some pretty bad things. So, Ben's reluctance to tell Luke the truth about his father's identity makes some sense. However, I'm pretty sure the same can't be said about why Ben doesn't tell Luke that his sister is Leia. As Leia's message plays, Luke stares at her obviously enamored. Ben even tries to convince Luke to come with him on his mission to Aldreraan, specifically emphasizing that they would be helping "her" (Leia) in the process, so telling him that he'd be helping his sister would certainly be helpful in convincing Luke to come along. Then, of course, there's the more obvious reason why Luke should know. Luke has the hormones of a 19-year-old, Leia is attractive, and putting 2 and 2 together, you'd have to assume Luke might be interested in hooking up with that cutie in the hologram. So, is there any in-universe explanation that could explain why Ben doesn't tell Luke that Leia is his sister? Help me fellow posters. You're my only hope!