The fact that some people are trying to excuse this is disgusting tbh.
Look, I don't have a problem with the portrayal of immortality and criminality in fictional medium. To do so would be absurdly hypocritical.
That said.. that said. This is not a happy story or a good ending. Let's not get it twisted guys. A happy ending would be both the MC's guardians struck off their medical licenses, a conviction for child abuse and handed a sexual offender registry because that's what this is yo. It's 2020 not medieval times, they know right from wrong just like we do and they have the same responsibilities.
We can enjoy this show as it is but to try and pretend like what went down was even remotely morally or legally excusable is just insane..
Anyway, still thoroughly enjoyed, despite what looked like a "this is all okay" message at the end. 7/10.
In addition to the "wrong, but ok in fiction camp" who have already spoken up, I'll point out there are those of us who, like Akira, think the specific situation as described would actually be ethically "ok" irl (that said a real situation that well justified is extremely unlikely).
Since this has been discussed at length earlier in the comments (and more recently the in-story reasoning from Akira), I'll try to be brief (... and after writing it, so much for that hope).
First up law/morality/ethics are all different things; legal/illegal isn't good/bad, even if that is the aspiration it can never perfectly align. I'm not going to be talking generalities here, obviously parent-child incest isn't a good idea in general (in the sense of not intrinsic/universal good), but that is not the same thing always being unequivocally bad.
There are various ethical frameworks used to reason about right/wrong, one that seems relevant here is consequentialism - judging the"rightness" of an action by consideration of its consequences.
"they love each other, what is the harm?" it is a deceptively simple, but powerful argument.
The crux of it is risk vs harm. Sure there are myriad potential negative risks associated with power imbalance, age gap and overlapping family-romantic bonds, but none of them are guaranteed to materialize as harms. They care deeply about each other, and Asuka is actively trying to get better at considering Ayako's perspective - probably the only negative outcomes they couldn't work out together is what other people might do to them if they find out... which by modern sensibilities is a lousy thing to base your romantic aspirations on.
Much of the uniqueness of this story is in the situation that has been carefully crafted to set up a situation where the "normal" incest negatives are absent or mitigated, to create the potential for a "good" mother daughter romance.
* Asuka initiated things, and has been the one pushing the boundaries (ie not pressured/groomed)
* Asuka has a clear understanding of her feelings, having struggled with them for years. (ie not confused/displaced feelings)
* Asuka has never wanted for affection. (ie not substituting romance for missing maternal affection)
* They are the centre of each other's life by choice, not for want of other opportunities. (not desperation/loneliness)
* Ayako has been trying to do the right thing. Her primary motivation is what is best for Asuka. She has been wracked by guilt - Akira got though to her by emphasizing Asuka's feelings. (not selfish).
Without all that context it would just be a titillating story about mother and daughter making out, but with it it becomes something much more interesting making a hugely taboo relationship relatable and maybe even ethically justifiable.
If Ayako had just jumped on Asuka and seduced her when she found her asleep on her bed at 14, it would have been a very different story.
btw as regards legality - since they have only kissed so far it would actually be very tough to prosecute. Up until the tongue kiss only their knowledge of the feelings behind it put it definitely outside the limits of acceptable mother-daughter behavior (how much kissy is too much kissy?), and even the tongue kiss would be more eyebrow raising than definitely illegal (sex crimes are usually defined around genital contact, and ordinary assault would require Asuka to be unhappy with the situation. Some jurisdictions have corrupting a minor/grooming offenses, but it would be extremely unusual to use them for a "victim" that old, Asuka is over 16, well outside the intended purpose of those laws).