MrJet had the right to create as hot a sister as he wanted, but I totally disagree with what you said about the provocative behaviour: if you have no intention of creating sexual content with that particular NPC, don't give her a provocative behaviour or put her in suggestive situations around the MC because this is a porn game and she is the MC's sister. Sisters don't act provocatively around their brothers in real life; it only happens in porn, and only for one reason.
The sister could have had a boyfriend (or not) and still could have helped the MC in every way that she did; she could have played a paramount role in the story's development without the nude scenes or the MC eyeing her up and down.
We have in-game scenes like this:
We have a promotional image like this one
including every fuckable girl in the game (except Catherine and Emma) plus Julia.
OK, not a big deal; it's just a pool pic and she's barely visible.
Well, in these special renders we can see her much more clearly:
There are a couple others, but these will be enough.
Maybe MrJet didn't do it on purpose. Maybe it never crossed his mind in the first place that people would be interested in the sister, and when they said how hot she was and asked for incest he didn't want to include it, but thought that some naked renders would suffice to appease them. Either way, from my pov it was a complete mistake, and as I said I had the impression that he was mocking the players.
PS: After a second look, I'm not sure if the pic with the sunscreen is original or fan-art. Anyway, I'll leave it there for everyone's enjoyment.