I'd wondered the same tbh, Ami's whole attitude towards him seems abnormal. Makes you wonder whether she hadn't been groomed for him by Sekai. Wouldn't be that difficult for her to do tbh, just to give her a nudge and a fixation.You know, i got wondering, if Sekai didn't die, what are the chances she might've egged Akira on to do things with Ami? She was 7, so it'd be later than i'd expect things to start if that's what she wanted, she had the hots for Akira when he was a baby and had planned to molest him all along, but i dunno, despite her love for her daughter she was an incredibly sick and evil person so if she even remotely liked the idea i have no doubt she'd have gone down that path... Ami did try to take care of Akira with sex when she first started living with him, but he was too comatose to accept her 'offer', at least if i read that one scene properly...
Honestly, I kinda wonder if Ami isn't his kid. But I don't know how the timeline falls.