I understand that genetic sexual attraction is a thing
You don't need genetic sexual attraction* to be a thing. You simply need them to not know they are siblings. Humans have no innate significant mechanism for detecting close relatives they didn't grow up with! There is no way for them to select *against* their siblings if they don't know them!
I agree that, as a filmmaker, I don't think Lucas had decided to have them be siblings when he wrote that kiss. But it isn't like it needs some deep explanation to handle. They were two people who had just met, who found a bit of an emotional connection in a stressful situation. Over time, that connection didn't develop. Leia went with Han (who, by what we've seen, looks more like Bail Organa!), so there's no real big deal.
I'm just making the point that it isn't like the Star Wars films have always been airtight and devoid of inconsistency.
I'd agree with that. Star Wars is not, say, B5, with a well-developed arc set before any production began.
*For those who don't know the term - it is the tendency for humans to be attracted to people who they have never met, but are genetically closely related to them. The name is a little misleading - humans have a tendency to be attracted to people who are phenotypically similar to the parent who raised you of the opposite sex. If you are a man, you'll tend to be attracted to women who looked like the woman who raised you. As I understand it, this holds even if you, as an infant, were adopted by parents of another race - a Caucasian male child adopted by Asian parents will tend to be attracted to Asian women. It isn't so much "genetic attraction," as an imprinting when you are young. I'm told this holds even if you are gay or lesbian - a lesbian woman has a tendency to be attracted to women who look like their fathers. In this sense, genetic sexual attraction really shouldn't apply, as Luke and Leia didn't grow up with their natural parents! Unless Luke looks like Bail Organa and Leia looks like Aunt Beru!
There is also a tendency for humans to be attracted to people who are somewhat similar to us - intelligent people tend to be attracted to intelligent people, musical people tend to be attracted to musical people, and so on. Both this, and the imprinting, are small tendencies, often overridden by other factors.
If you *know* your close genetic relatives, there is an imprinting that has the opposite effect - if you grew up with your siblings, you'll tend to not be attracted to them.