This is the beginning of the revelation I guess. This whole chapter is a flashback really. Sorry for the wait.
Disclaimer: Heck no way do I own Katekyo Hitman Reborn, I'll never be that pro.
Chapter 4
The silver falcon: a name given to the girl assassin whose weapon was always silver and whose hunting instinct was parallel to any bird of prey. Most people didn't know her name or her background. No mafia had ever claimed her, she had simply appeared at the age of 12 and began stealing mafia jobs. Her specialty, as far as the people researching her could tell, was information gathering and assassination, in other words, things in which stealth is most important. Because of this tendency to avoid straight-out fighting, Shiyo's name was not very well known in mafia circles for a long time, but that was the way she liked it. In fact, she didn't think ANYONE in the mafia world knew her name. She had never volunteered it, and anyone who had known it was long dead.
When Shiyo had explained her birth to Tsuna, she'd done it in a both concise and forceful way, leaving no room for doubt as to the truth of what she said. In actuality, however, it was complicated and confusing. The reason she had been taken instead of Tsuna, the way in which her life had played out, her life and understanding of the role she played in it, none of it was simple.
Shiyo's life started in confusion. When she and Tsuna were born, it was not a well-known fact in the mafia world, but that was because they weren't especially important. Admittedly the twins were the children of the external advisor of the Vongola, but they were not in line to become the bosses of the Vongola. However, it was known by an ex-mafioso who lived nearby. The ex-mafioso living nearby were actually a family as well, and coincidentally, the woman had a baby the same day Nana had the twins. The child she gave birth to, however, was a stillborn baby girl.
This stillborn was obviously not Shiyo, but the unnamed child's birth, or rather, failure of birth led to Shiyo's stealing. The father, mad with grief, began looking for a replacement and somehow remembered that the Vongola external advisor's wife had just had fraternal twins, one of which was a girl. And he switched the two children. Watching the family carefully, he found a perfect opening and simply switched the girls.
He never told his wife what had happened, but he did write it down in a secret journal he kept in a tightly guarded safe. They moved away from Namimori and raised Shiyo in Tokyo as if she were a mafioso. Every day she trained her body and mind strictly and it wasn't long until she became the smartest, most efficient, and strongest girl in her grade.
A man's past will always come back to haunt him. The fear of that maxim was beaten into the lives of Shiyo's so-called parents, and one day, it came to pass. While Shiyo was in school some time in the winter, her parents were assassinated.