SPOILER The Huxes and FN-2187...

Correction: There was an "almost" kiss before Han showed up that was more passionate. 3PO interrupted before they could actually kiss. She wasn't just kissing Luke to mess with Han as in the surviving scene.

I don't see the kiss in the film as incestual because Leia clearly did it to mess with Han. Luke's reaction can also be seen a goading Han. Though Lucas actually had an attraction between Luke and Leia when they were cousins, so...

I remember an interview years ago with Lucas where he said he edited that original scene out because he had decided to make Leia the "other" by post production. I'll have to dig more to find evidence. But with Lucas, who knows?
I know about the original intention to have Luke search for his sister in ep7. I've posted about it on the site multiple times. Lucas realized that he didn't want to make 7, 8, and 9 (or by other accounts just didn't have enough story for ep6) and decided to fold those ideas into ep6.

EDIT: I do think Leia hearing Luke may be evidence of her being the "other," though I wouldn't die on that hill. It is interesting that Vader contacts Luke in the same way at the end. "Leia doesn't have to be force sensitive or a direct relative to receive telepathic communication from a skilled force user." The only other telepathic communication we saw in the film was from Vader to his son. Unless you count Obi-Wan speaking to Luke, but he was a Force Ghost and was speaking to a Force user.
We know Lucas had her as the sister by at least Jun 12, 1981, the date of the Revised Rough Draft of ROTJ.

 

If you are speaking about speculation that Finn is a Leonis, Zare is at least a new character and it gives Finn a ready backstory for his lineage if he discovers it. It isn't far fetched that they would plant this in the extended canon.
If he is related to Lando, then it is kind of silly, though it wouldn't be that crazy to think the offspring of close friends would interact during important events in their lives.
I think people simply have fun thinking about the next generation of these families being a part of the new films. And to be fair, many are grasping at straws trying to guess who if any of the new cast are related to the old crew. It doesn't mean that they literally think every new character is related to an old character.

 

I think I get what you mean now, though I don't see how the order of when the names were revealed has anything to do with it. We don't know when the characters were named behind the scenes. If they created the Zare Leonis name first, there is nothing to suggest that they didn't plan for that character to be the father or uncle of the "Finn" character from the beginning and then name that character Finn to work with Leonis (unlikely), or that they decided when creating Finn that the existing Leonis character could be used for his backstory and named him to fit well with the Leonis last name (also unlikely).
And just because Finn's name was revealed after the Zare episode doesn't at all mean Finn was named last. I believe Finn was named before we ever heard of Zare and it is possible that Zare was added to Rebels to be an existing backstory for Finn. If Finn finds out his last name he could still go by Finn Leonis (he might not get a real first name revealed to him, especially if his parents aren't alive), thus making it necessary for the creators to make the names work together.