Alexrd said:
If they hadn't shown Owen and Beru, the audience wouldn't understand how are the Lars and Anakin related.

As folks have pointed out, the explanation found in pre-PT-EU (oh wow) was that the Lars were relatives of Obi's, and Luke just called them Uncle and Aunt either because he believed they were or because that's what people call each other sometimes. It worked. I can easily imagine Obi-Wan saying "I have a brother on Tatooine who might be willing to take him in.", though I think an earlier mention of this would be good.

That being said, the current setup has never bothered me. Anakin has dreams about his mom. He tracks down his mom. His mom is married. He has a step-brother. None of that is implausible. It's really just the part where they decide to put Luke on Tatooine at all that irks, but... well, if there's a bright center to the galaxy, it's farthest from it.