There's no conflict if Helen, Leia and Katie are all related. Katie could simply be the jealous little sister who caught Leia with her hate fuck buddy who threatened to out them to the groups mother figure, who is also their mom. That's a toxic relationship, more than enough reason to prevent William and Leia from risking a pregnancy. It's not complex. This whole "Zane bloodline" theory is contrived and doesn't exist in either story at this time.
SG is REALLY simple to understand. It's a slice of life story with two children who grew up together, were orphaned at the same age, but share no biological relationship. Sitcoms with similar scenarios existed from the 70s to the 90s. People are confused why they look so different bc they THINK that Nika and Nami are related and it's addressed repeatedly. They don't even have to come from the same city. You literally have to go out of your way to inject incest into that story to see it at this point. Even if you proceed down Nami's path, which goes no where currently, they are still not related. Certainly not biologically.
Even if you want to argue that if they were adopted, they are legally related, that is never addressed. Personally, I find it ridiculous that if you had two identical households, one where the children adopted and the other fostered, one would be considered incest and the other not. And it doesn't matter bc YOU have to make the leap to it being something perverse for it to be something perverse. That only says anything about you. And, mind you, we're talking about a fantasy incestuous relationship which doesn't even exist in the story itself. And, unlike WiAB, SG has progressed further than it originally had before the changes. So you're making a massive fucking reach in the best of circumstances. At worst, you're on a forum arguing semantics to defend your fetish from non-ridicule, which is desperate and pathetic.
Now, why would Nojiko NOT want the two kids she's raising to suddenly have a kid of their own? The question answers itself. It's the same reason parents don't want their own kids having a kid with a non-sibling.
You're seeing 2+2= and 2+X=4 and coming up with 5 and 3 as answers. You're making leaps beyond the evidence you're being given. Sure, that's enough to satisfy some peoples fetishes w/o going too far, but it's a leap they have to make in their own minds. The rest of us can see it, roll our eyes, and continue with the story as it is being presented to us. The only conflict exists bc one party is determined 2+2=5 and everyone that says its 4 is wrong. Make your leap, it was left ambiguous to allow you to do just that, but stop demanding everyone else make that leap as well.
