The idea of two unrelated close friends who grew up together feeling that a romantic and sexual relationship isn't straightforward, and something they can just be open about wanting, is not new and does not originate from adult games. That they might want to keep it secret is not surprising nor is it an indication they are supposed to be siblings.
The people I live with who are not related to me are my housemates. This is an extremely common term and not automatically a secret code for sibling. The single use of this term in Love Season is immediately followed with multiple lines about Nova's origin outside the MC's family and how she came to live with them.
Likewise, "bro" is a common affectionate term used in some cultures between people who are undeniably not related. MC calls Marcus bro. A random blonde person at a party calls the MC bro. Are they both his siblings? The only instance of "bro" between related people is Brooke calling Trevor bro.
Maybe some people want so much for there to be incest they will clutch at any straw to avoid admitting that there is no reliable evidence that incest was intended and solid evidence against. I imagine they also either haven't played the current update or are in denial over the scene with Alice, Nova, and the MC. Whatever "evidence" you might think you've found before, the author has now clearly spelled out that Nova and the MC are not siblings. It even addresses the reasons why they don't want anyone to know about their sexual relationship.
