Akira is 31, Niki is 29, Ami is like... 15? so Akira was 16 years old when Ami was born and 15 years old when Sekai got pregnant. I suppose it's okay here.Kyoto takes place roughly a year before Ami was born, which would make Akira around 12, and Niki around 10.
For Noriko: in Rain King we saw Sekai with young Ami and Ami remembers her mother, so Sekai didn't die when Ami was a kid. So Noriko was a kid too and Akira could easily be with Niki at this point. Tragedy will struck later.