1. "I'm not much more than an interpreter, and not very good at telling stories..." C-3PO to Luke (ANH)
Well, we know that Threepio can tell stories, and make them interesting. As for "not being much more..."? Have you taken a trip lately to the Ewok village, where, for some strange reason, they worship a five and a half foot tall droid instead of the much more obvious choice of Chewbacca? WHAT?

2. "I believe she was a passenger on our last voyage. A person of some importance as I recall." C-3PO to Luke, in reference to Leia.
Oh, please! Do you mean to tell me that a ride in an escape pod, a walk through the Dune Sea, and a jawa restraining bolt will erase memory? Bantha poodoo! Am I wrong in rembering him saying, quite clearly, that there would be "no escape for the princess this time?" I think I've made my point.

Now that you see where I am coming from, I will throw my hat into the speculation arena. Just a note that MY speculation is, likewise, based on speculation, so this may all end up as chow for the Dianoga, but here it comes anyhoo...
Artoo and Threepio are owned by Anakin Skywalker as a young boy in the prequels, then Artoo & Threepio knew that Vader was Luke's dad and never saw fit to let poor Luke know it. While you can laud their secret keeping abilities, why didn't they (or one of them, at least) tell Luke? When Obi-Wan was dishing up his load of Mynock droppings about Vader betraying Anakin they just sat there running a self-diagnostic or something. Is that loyal? No.

I think that they must have also known that Leia was his (Luke's) sister. How else would her dad's old droids end up serving her? By chance? I think that 3720-to-1 wouldn't even come close to the odds on that one. Threepio would have to dig through all of the more than 6 million forms of communication to find a believable excuse for that.

I know that there will be those of you who feel the same as I do, and those that don't. I'd like to hear from you. One way or the other, send me a note why don't you?