i made that particular edit. and a number of others.
how it came about? on /co/ somebody did it as an exploitable... all the text removed and just Katie's "ewww" left in, so people could add their own dialogue. a few people did different variations, and i was one of them. i found it really fun, so I went a little overboard, including a lot of different pages from the comic and sometimes editing the actual art (for example, there's a series of the 'eww' comic with Katie talking to adult-Julie instead of adult-Katie that i made, and a few other variations).
as to why incest?
well, i LIKE incest. there's no specific incest hinting in the series that i'm aware of (although there were a few times Julie was naked in front of her siblings... there was a plausible explanation each time... it happened most often when she had the cloud powers, which left clothes behind if they weren't her special costume, but it's a bit eyebrow raising), i just think they make a good incest couple, and apparently others agree since i've seen a few other incest edits, porn, fictions, and other such things over the years.