Actually, now that it's come to this, Akira might make a surprisingly strong addition to the story. I know people hate love triangles, but the biggest tension in Ayako and Asuka's potential relationship - okay, the second biggest tension in their potential relationship - is that Ayako may be looking at her daughter as a substitute for her husband, right? Asuka has half his DNA, etc. Well, if that's her motivation, then doesn't Akira serve that purpose much better than Asuka ever could? Not only does she share much of the husband's DNA, she shares Ayako's pain of having lost him, she's an adult, and, y'know, she's not her goddamn daughter. That's (usually) a bonus.
If Ayako wants a replacement, Akira is eminently the best person for the job in every way. If Ayako chooses Asuka anyway, then it'll be because of Asuka's unique qualities as an individual, and not because of her similarity to her father. Or it'll be because Ayako secretly has a massive incest fetish and just thinks it's totally hot to get it on with a family member, but shhhhhhh, never mind that.
This is a really great analysis of this development. Usually, introducing characters this late in a story is really bad, and still kinda is here because there was no foreshadowing to Akira's existence whatsoever. However, as you've said, her introduction is fortuitous and is the perfect dilemma we need at this stage of the narrative.
Personally, I hope it does end in incest. I mean, that's what the story is catering to and it would feel disingenuous if their relationship was thrown away for the late-entry college sister-in-law. Also, I don't know if Asuka could handle being a part of the family after having been accepted then later "dumped," for all intents and purposes.
I said this earlier, but we're all going to hell for reading this.