First of all, undo the "I...am your father" revelation. In my ROTJ, Vader and Anakin are two different people; Vader, having slain his former comrade, has developed a delusion over the decades that they were the same people the whole time. When Luke and Obi-Wan talk about the confrontation, we are informed that it is in fact Vader who lied, not Obi-Wan.
Leia and Luke are not related, but neither are they attracted to each other beyond that of a strong friendship. In a sense, their relationship is indeed sibling-like, but without the embarrasingly tacked-on blood relation.
Why? Consider Luke-Vader II. When Vader says, "... if you will not turn to the dark side, then perhaps she will.", Luke reacts with anger. I'm not so convinced in that moment; Vader has been playing against Luke's feelings for Leia as his sister. Luke has strong feelings for Leia, but they've only been brother and sister for like 20 minutes. This person who I have not known as a sister should not produce such an emotional trigger in that way. In my ROTJ, let's up the ante a little:
After the sabotage team lands on Endor, a skirmish with the Imperials ends with Leia held captive. She is taken to the Death Star, a prisoner of Darth Vader yet again. Han and Luke have an emotional debate over how to approach the situation; Han wants to break from the team and rescue her personally - after all, they are in love and she freed him from the carbonite. He feels personally indebted to her, similar in a way to Chewie's life debt. (Although this is strictly an EU conception, I like the Chewie life-debt thing) Luke argues that Leia would always think first of the mission, and that only a Jedi knight could infiltrate Vader's DSII stronghold, just as Kenobi ventured out into the original DS. Han reluctantly accepts, though he knows Luke is right. In this way, we see Han's arc work more thoroughly - when he is convinced to stay behind and fight, he fights not as a fun-loving space cowboy - he fights with the burden of a man committed to fighting for others. (Besides, this turn of events would have given Harrison Ford something to do)
Luke Kenobi's his way into Vader's stronghold. There is no Emperor, there is only Vader and an unconscious Leia secured to the wall. Vader kidnapped Leia knowing that the act would lure his "son" to him. Vader indeed plans to use Luke's considerable powers to tighten his grip on the galaxy, but if he cannot persuade Luke to join the dark side he will eliminate Luke as the only person in the galaxy capable of opposing him.
The duel is evenly fought early on, but Vader goads Luke towards the dark side the entire time. He reminds Luke that the dark side flowed through him in killing millions on the Death Star. As he slew every one of Jabba's henchman. As he struck down the faux Vader at the tree. Luke responds to this as a Jedi would, shifting himself into a purely defensive mode. Eventually, Luke powers down his lightsaber - he understands what Kenobi had done against Vader in their last duel and tries to do the same - and hops onto a catwalk, just as in the real ROTJ. In the same way as the actual movie, Vader hurls his saber at Luke. Luke's light side mastery is not yet so complete as Obi-Wan's, so he saves himself with a last second dodge. The flying saber destroys the catwalk.
Luke's reasons for rushing to face Vader centered around his friends - and as Vader searches for Luke in the shadows, he invokes Luke's closest friend in order to draw him out. If Luke will not fight, then Leia will suffer unimaginably. Vader begins to describe Leia's coming ordeal when Luke leaps from the shadows and overwhelms Vader, reducing him to a smoking, immobile, helpless heap. Luke is on the edge of fully indulging in the dark side when a barely awake Leia manages to appeal to Luke's better angels. And the circle is closed. Luke, Han, and Leia have already saved each other many times throughout the trilogy, but this intervention of Leia's is by far the most meaningful. Luke would certainly have filled the power vacuum left behind by the slain Vader.
As Vader lies in ruins, Luke and Leia escape in Vader's personal shuttle moments before Lando's attack claims the DSII. In the final scene, Han and Leia reuinte with the fervor of true lovers aware that they might not have seen each other again. Hugs all around for everyone else. Luke sees three Force ghosts: Obi-Wan. Yoda. Someone else. Luke looks astonished and slowly mouths the word, "Father?" (remember, there is no dialogue) Anakin Skywalker mouths in return, "Thank you, son." (The implication here is that Darth Vader was working to keep the spirit of Anakin Skywalker from communicating with Luke and other Jedi)
Oh, and what happened to the Emperor? That's for the next three movies.
You are entitled to your opinion but not having Luke vader's son makes zero sense. It only makes sense with 1977 star wars but the reason luke has darkness in him and is so succeptable to temptation is the weakness of vader in his blood. He would not go all dark were he the son of the Perfect holy man skywalker lucas originally envisioned. Unless he entire motivation is the murder of both his father and kenobi. Even so a he would not just throw away his discipline and ideals so loosely. The Jedi ideal is against revenge. Revenge is selfish and not selfless as a jedi should be.
Would he so easily betray kenobis and his fathers ideals he would be an even weaker character, without moral fiber and too easily led or pushed to do things. Evil and its agents cannot force you to do anything under free will unless you go along willingly or are slowly tainted by corruption without yourself noticing you have betrayed your own better nature and ideals.
Vader/Anakin was originally tempted by the dark side i would like to believe because of the desire for power to do good at least at first. To order the cosmos according to his ideal protect those he loves and cherishes and punish the wicked. In the end the ends justifying the means rob him of his humanity he becomes a servent of a machine government that crushes people beneath its Steel boot and holds the galaxy in its iron fist.
He was never free until Luke saved him on the death star II up until then he was always a slave. First a slave as a little boy on tatooine. Then a slave to his ideals and the jedi order and a servent to be called on by the people of the republic in selfless sacrifice. Then a servent of a tyrant and the evil sith. Only in death is he freed from his burdens. He realizes to late that family above all is more important than anything else.