I can't remember if I suggested this.... but.....
What if LEIA DIES?
Luke learns about leia as usual from obi-wan. He has the same conversation with leia later on, but does not tell her about being his sister (assuming he is going to die himself he decides to spare her the pain, or perhaps just decides that it is too unbelievable himself). Vader still uses the knowledge against luke later on, and luke beats him etc. But the big emotional crash at the end is that he never gets to tell her the big secret, and has to carry it himself FOREVER.