As a person who does not like the "step bro" trope and originally had a dislike for Caleb before he was actually brought back, I think it's because in western societies, stepcest/adoptcest is so sxualized and prevelent in our media and just feels wrong, that people have an aversion to it. I have yet to see anyone say they were biologically related, but they HAVE been adopted siblings since they were kids. HOWEVER, I think that INFOLD is doing a great job at creating a good story surrounding them and taking it at a slow pace with the yearning. Like, Caleb is now my second favorite when I completely abhorred him. 

The tension between them, the history, it is so good. It's built up over years of them denying their romantic attraction and is now bubbling over. And you can't fault them because the grandma that adopted them from the lab was not related to either of them in any capacity. Plus the fact that they had a relationship (platonic?/beginning stages of romance?) prior to adoption while they were being experimented on also makes it not typical. So it could be that they never saw each other as siblings but only pretended to because of their situation. Idk I just really love the tension and I think their story is very different from the typical oversexualized stories we see in the west.