
I cannot help but see some parallels between the kidnapping of Akira’s sister in SMT IV and Asahi’s death in IVA.
Both girls are taken into an ovoidal vessel that would ferry them along with countless others to another world as part of a god’s grander scheme. Akira’s sister is kidnapped as one of God’s Chosen and holed up in a giant cocoon, which in the main timeline did succeed in flying her off to what would eventually become the Kingdom of Mikado. Asahi was killed and her soul became one of the many others trapped in the Cosmic Egg, which would have led to the birth of a new universe that all the souls would be reborn into. Different circumstances and outcomes, but still the same idea.
There’s unfortunately nothing that states or even directly implies that Asahi is or was ever intended to be the reincarnation of Akira’s sister. But when considering her intrinsic closeness to Nanashi as foster siblings, how Nanashi is always made to keep an eye on her, and even just the parallels between the cocoon and Cosmic Egg, I think there is a reasonable degree of implicity that Asahi is or could be the reincarnation of her. At least, even as pure fanon, it wouldn’t be a far-fetched one.
Akira succeeded in reaching the Firmament, but because of the Tokyo-Mikado time dilation and the years it took to even dig through the ceiling, he never got to reunite with his sister as she would have already died a long time ago by then. The DLC timeline note in IVA’s database states that 300 years had passed on the surface by the time Akira breached it.
In a way then, Akira and his sister are vindicated when Nanashi enters the Cosmic Egg and rescues Asahi in the Bonds route. Time is a flat circle.