i always imagined that uncle owen was not related at all, but just a random guy that obi-wan chose to keep luke, telling him what the deal was with luke just so they'd know what they were dealing with, and basically having him do his best to keep luke in the rural life. but then when obi-wan realizes lukes potential and the certain fate of his adoptive parents, he tells him a fable about his unlce and his father being at odds about involvement in wars, helping luke to side with his adventurous nature. but thats just what i imagined, and it seemed cool.