WARNING: Long post is longgg...
I dunno if anyone has come up with this before but i think that it is possible that Tatsuya & Miyuki are actually cousins rather than siblings...
The whole idea first came about when i read vol 8's "Untouchable — The Nightmare of 2062", where it was revealed that Yotsuba Maya was kidnapped and was
And my body has been messed with all over. Even the inside of my body has been disturbed. There is no place in my entire body that has not been fouled.
Thus it is down right possible that eggs had been harvested from her body during that time.
And even though Yotsuba Genzou wiped out the Kunlunfang Institute & Dahan in revenge for his daughter, there is no guarantee that he had managed to destroy every sample that had been taken from Maya. Furthermore, of the 7 Sages, one of them, Jiedo Heiga, is a survivor from Dahan which means either him or some other survivors could have taken the eggs/genetic material & gone underground more than a decade before they restarted/started their experiments. In fact, the reason why Tatsuya is a Born-Specialised Magician could be due to these experiments...
Okay, now that we got his "conception" cleared, how did Tatsuya end up with Shiba Miya?
I believe this is where the Kuroba, a Yostuba branch family that are responsible for the Yotsuba intelligence network, comes into play. They managed to come across information regarding the experiment and informed the current Yostuba head, Yostuba Maya. Either Maya, or someone picked by her, would have annihilated the lab & brought Tatsuya back to Japan. And Maya didn't want to raise a child that she doesn't regard as her own, so thus she sent Tatsuya to her married sister to be raised as Shiba Miya's "son" & Miyuki's Guardian. Plus, it was known that Maya was barren due to what was done to her during her kidnap and it will be hard to explain the sudden presence of a child in the Main Family.
This could happen at any point of Tatsuya's childhood but personally i believe it should be at some time shortly before he turned six where the experiments performed on him by his "mother" & aunt was supposedly to had taken place.
Now, it has been scientifically proven that a person learns both how to understand & express emotions in their childhood. But what if their childhood experiences had been erased & replaced with data-like memories? Won't the child be unable to process/express emotions properly? Just like Tatsuya? He does not have the ability to feel strong emotions, in fact he fakes most of them just to be able to fit into society.
The only true strong emotion that is left within Tatsuya is his love for Miyuki.
As for how Tatsuya's original memories were erased & replaced, that should have been done by Shiba Miya.
When she was 12 years old, she had been unable to erase her sister's memories of her traumas, she could only turn Maya's memory into knowledge. In guilt,
Miya broke her body from overusing her mind manipulation magic before she turned twenty like a penance.
During all those years, Miya could have finally found out how to use her magic to erase memories & thus able to use it on Tatsuya. However, implanting new memories is a different kettle of fish altogether, which should be why Miya could only implant data-like memories that are closer to knowledge rather than real memories that are experiences.
Last but not least, i noticed that for Japanese dramas/manga/anime etc... Whenever the authors want to make a bro-sis pairing work, the deus ex machina of "he/she is adopted! So everything is okay" would be used. Which is why i feel that it is possible that Tatsuya & Miyuki are actually NOT siblings...
So... What do you guys think?