Where is it written in stone that Owen does not recognise C-3PO?
He wants their memories erased ASAP, as far as I recall.
As soon as the name Obi-Wan comes up in conversation, I'm sure his fears are confirmed.
I don't see this need for everyone in a film to say what is on their minds: "You - protocol droid! I know you. You're that one who was built by Luke here's father. Y'no - Luke - the boy who I am trying to protect by making sure he never follows in his father's footsteps or learns the truth about what happened to Anakin and........ damn! Did I say all of that out loud? God, I'm stupid!"