daneditor said:
Up until this week Leia wasn't Luke's sister in this edit - the problem is, why does Luke finally attack Vader if it's not about Vader mentioning/threatening his sister?
daneditor and I have been bouncing some ideas around about how to solve this little problem. I believe his original intention was to remove the sibling subplot completely; after all it is called the Radical Re-Edit.
IMO Lucas never should have put the subplot in the story but rather should have developed Luke's story in more depth or added more to the Han/Leia story besides Han's jealousy. The whole Luke/Leia thing comes off as an afterthought and just doesn't seem to pay off.
The key to solving this problem is already in the film: The Emperor causes Luke to lose it, not Vader. Remember the Emperor spends 20 minutes goading Luke about how his friends will die and then starts vaporizing Rebel fighters. After that he tempts Luke to strike him down with all of his hate, which he attempts to do. So here we go:
I've made my own clip to demonstrate how this could work. I've shown this to daneditor; he liked it but wanted feedback from others. So here it is, let us know what you think: