Unfortunately it will have to be assumed Obi-Wan contaced the Larses before dropping Luke off. Some set up of the Obi-Wan/Owen animosity would have been nice, but we have what we have. Owen being Anakin's step brother was a clever move by Lucas to actually make him Luke's uncle (the poor kid had enough lies in his life) without giving Anakin any siblings which would no doubt complicate things. Owen being Obi-Wan's brother is a ridiculous idea that never made sense to begin with.