Luke wouldn't have been surprised when Vader told him that. He could have used the moment to pretend to be shocked, and then kill him.

According to Leia in ROTJ - She ALWAYS knew Luke was her brother, lol

Changes -- IMO

Leia might have attempted to tell Vader who she was in A New Hope, but i'm not sure Vader would believe her. he would however bring her to the Emperor to attempt to turn her, and since Leia was not trained as a Jedi she would have been by the Emperor - causing Vader to be "pointless" in the Emperor's eye.

Without Leia the Rebels are not as strong as they are in the "real" universe, also because Leia told Vader who she was BEFORE the destruction of Alderaan that planet is not only still around but is "under the protection" of Palpatine (until Leia is fully trained) - without Alderaan's destruction a loot of people DO NOT join the Rebels making them even weaker of a fighting force. and without Leia would anyone even know if about the Death Star Plans inside R2-D2 (she told Han about them)

After a few years Leia is trained in the Dark Side enough to be useful (around the time of ROTJ) the Emperor (as a test) orders his new "Queen" to destroy Alderaan, and she obeys wanting the Power it would give her (both as a Sith and as Palpatine's right hand) because of her loyalty Palpatine would attempt to kill Vader who escapes and would switch sides to stop him and save her teaming up with Luke.

in the end (sometime after ROTJ) Vader dies at the hand of the Emperor (force lightning), Luke fights Leia (who is now trained thanks to the Emperor) and she kills him - in her lust for Power SHE KILLS the Emperor becoming the head of the Empire

You'd hear Sweet Home Alabama a lot more.

Well that would be a different story

Plus the biggest reveal off all time in movie history would never have been made.

So that would have been interesting

Their kiss would be pretty weird then