I'm not sure if Lucas wasn't rolling the idea of having Leia, Luke's sister in the back of his mind even back then.

If you watch ESB there are a lot of clues dropped in there to suggest it might be her.

As for the smooching Lucas is a bit unconventional when it comes to that sort of thing.

Marion and Indy's backstory, little Anakin and teen Padme...

The histories of myth and melodrama is full of lost siblings in love trapezoids which are collapsed by the revelation of the truth.

It happens in real life too.

I don't read Yoda's line as referring to Leia anyway.

To my ears he is clearly talking about Anakin.