CT Are deleted scenes pertinent to Star Wars Lore

Which actually works OK - just. The extended version clearly points to a genuine romantic interest between the two - something which, at the time, was an intended possible outcome, and another dramatic point - but without it, the actual 'kiss' between Luke & Leia in ESB merely comes across as a spiteful gesture by Leia towards Han, with Luke being merely incidental. She might as well have snogged Chewie - except that Han wouldn't have been as jealous if it was Chewie. At the start of ESB, Luke was his romantic rival for Leia's affections.
Yes, it's her brother, but she doesn't know that, he doesn't know that, blah blah blah.

It wasn't planned that the romantic relationship between Luke & Leia would be rendered so meaningless and 'innocent' so very quickly, but like so many other such incidents across the OT (which was being made up along the way), GL got lucky when the medical bay scene was shortened to eliminate what is now a far more creepy near-kiss between the siblings. If that had made the final cut, I doubt he'd have gone with the Luke-and-Leia-are-brother-and-sister twist altogether (which, BTW, I've never believed was necessary anyway) - but he'd have then been forced to address Luke abandoning whatever feelings he had for Leia, or perhaps jealousy towards Han.

The Sister Leia twist has always been regarded as a bit clumsy (as a cheap way of resolving the love triangle and an even cheaper way of dealing with the 'Other' Yoda mentions), but now that I think about it, it was probably a better alternative to the sort of messy nonsense required to resolve whatever feelings Luke had for Leia, without the story really continuing any further.
With Luke & Leia as brother & sister, all three are able to reunite at the end of ROTJ in a dialogue-free montage, simply hugging and exchanging meaningful looks. Without it, the free-flowing end of the film would have had to be interrupted with some laborious crap like this:

Bleh. Seriously, try to write something better that would substitute for that three-second-long heartfelt bear-hug and look of genuine man-love that Luke gives to Han at the end of ROTJ. It all worked out in the end.

Continuing with a rivalry between Luke & Han for Leia might have worked if they'd continued with the 'Adventures of Luke Skywalker' vision for Star Wars (open-ended series, or 12-episode serial), but that basically died around 1978/1979 when the backstory of Father Vader was invented and it became a dominant force upon the existing story - transforming the lighthearted space serial into an epic Saga, complete with three entire untold Episodes looming over proceedings.