Told you, did he?
Unexpected this is, and unfortunate
Why did Yoda think Darth Vader would not tell Luke? If they thought it was a secret to Vader, they did a bad job hiding it since Anakin and Luke have the same last name. So, they knew he would know and certainly it would be likely that he would tell Luke the truth.
Why would Yoda(and perhaps Obi-Wan) think it is unexpected that Vader would tell Luke?