‘Whats the problem with mothers and daughters being together?’ Well let me think...EVERYTHING
Like what? Be more specific.
The inherent power imbalance between an authority figure and the person they have authority over (one could argue the power imbalance between parent and child is one of the biggest possible depending on their dynamic), the issue of grooming a minor (for example via sexual abuse as a child). Like there are reasons incest is taboo beyond fear of birth defects that aren't just squeamishness and a lot (likely the vast majority) of close family incest is sexual abuse.
Those aren't things intrinsically wrong with mother/daughter incest, just bad roads that lead there... they could apply to non-incestuous pairings too... and don't really apply to this particular case. Even power dynamic is blurred since Asuka is arguably more self-possessed and (emotionally) experienced than Ayako is (and if anything Ayako's lack of sexual interest/activity would have been the opposite of grooming).
There is a difference between something being risky/unwise, or commonly associated with wrong-doing, and actually being wrong in and of itself. Incest isn't something people should aspire to, but if they happen to find themselves there (consensually) it doesn't have to end badly (and even if it does, arguably the same applies as to non-incestuous relationships... it is their mistake to make).
Obviously they have to be careful and considerate about how they proceed, but it appears they are both capable of that.
Stats wise, the true incidence of incest is hard to determine because people keep it secret, but one dataset that comes up is a survey of New England college students (Finkelhor, 1980) where 15% of females (10% males) reported having had some form of sexual contact with a sibling... the researchers' analysis classified only a quarter of those as exploitative, ie three quarters, one might say, "the vast majority", where non-exploitative. It is likely exploitation is a higher proportion of parent-child interactions, but sibling interactions are thought to be much more common, so it is probably safe to say most close-family incest is innocuous. You only hear about it when abuse occurs (likewise could conclude most heterosexual interaction is rape since that is what you see most on the news).
One of the top qualifications for a taboo is that it is something most people don't want to do themselves, and can't understand others wanting to do. This allows people to identify taboo breakers as "other". eg the massive taboo over homosexuality (past, and even into the present in many places), or BDSM, transgender, etc.
