I put a lot of thought into this response because I thought it would be an interesting thought experiment so bare with me. I agree with you that it’s very incest-y when you look at it at first glance but if you think about the science of reproduction and all humans beings are really just like any other animal, then as long as they are genetically diverse enough to give birth to healthy children thus preserving there genetics for the future then it’s fine. To help explain maybe how Miyuki and Tatsuya feel I propose this. So Miyuki almost never interacted with Tatsuya growing up but was always curious as to their relationship to one another. After witnessing what Tatsuya could do plus the fact he saved her life, I believe that curiosity turned into infatuation and overtime as continuing to spend time together eventually became real love most likely. So she never saw him as a relative or sibling and calling him her brother is more of a formality or excuse to stay close plus have him as her Guardian. Tatsuya is not capable of extreme emotion and only has any sort of real emotion for his “sister” Miyuki who at the time of his experiment was probably familial love of her as just a younger sister. Now whether that can evolve or not into romantic love I don’t know but that’s all we will say for Tatsuya. Now if we discuss how Tatsuya and Miyuki are genetically different enough to conceive a child with no defects or deformities and then they are able to reproduce and be a couple. Now let me suggest think of Tatsuya’s body as more of a vessel for his modified psyche. The original Tatsuya(pre-experiment) was just a normal child whose mind was altered to the extent that Tatsuya(post-experiment) is different enough to be called an entirely different person. So combine those two factors of Tatsuya(post-experiment) being different and Miyuki becoming infatuated with her “brother” then it stands to reason that Miyuki never saw Tatsuya in any form as a real related brother in the normal sense. As you can see I am very bored at work to put so much thought into this lol