Yeah, the "adult is the murderer of the child' metaphor is fairly common in literature, but it's not something people use in everyday conversation, which is what the dialog in a movie supposedly represents. Like I said, even if Obi-wan is just trying to skirt around the truth, it really doesn't reflect well on him, or on the Jedi in general. Yes "your father is a mass-murderer who turned to the dark side of the force and now calls himself Darth Vader" wouldn't be a nice thing to hear, but keeping Luke's parentage a secret from him is leaving him open to be pretty badly blindsided in the future.
The twin sister bit is also easily explained, Leia shows no signs of being a Jedi, Luke does. It isn't until the expanded universe that sorce sensitivity in Leia is ever mentioned let alone developed as her being nearly as powerful.
Not that it's particularly relevant, but that's simply not correct. Here's what the ghost of Obi-wan has to say about Leia in Episode VI, after Luke figures out that she's his sister:
She hasn't been trained in the ways of the
Jedi the way you have, Luke ... but the Force
is strong with her, as it is with all of your
Anyway, like I said, these are relatively minor nots compared to the problems trying to tie the prequels and the originals together (for example, nearly everything Obi-wan says to Luke in episode IV makes no sense taken in the context of the prequels), but it's pretty clear that the original movie was written as a stand-alone, and the plot twists in later movies were not planned for in the original.