I have been saving Shattered Empire for after I watch ROTJ in November as part of our "journey to TFA," but I'm very curious if Owen and Beru are depicted as being aware that Anakin survived and became Darth Vader. I believe the Legends consensus was that our heroes at the end of Episode III took Anakin for dead, explaining why they didn't do a very thorough job hiding Luke. Obi-Wan didn't find out that Vader lived on until he saw it on the news one day at the bar, and I presume he just told Owen and Beru that Anakin died in the purge. (He may or may not have mentioned Vader since, again, he believed he was dead.)

Even if all this is the case, it still would be possible for Ben to tell them the truth later at some point. But to me, Owen and Beru's conversation in ANH makes much more sense if I suppose they believe Anakin died along with the bulk of the Jedi.