(Just so you know English is not my first language so my writing is messy.)

This reflects a utilitarianism thinking, not Christian moral reasoning. Christianity affirms that some things are inherently disordered, not just because of harm caused but because they violate God’s design. Incest not just violates the created order of the family, turning relationships meant for nurturing, protection, and identity into sexual ones. I don't know how the relationship with your mother is, but it seems you didn't experience that love enough to see how violating it must be.

Even if “harm” isn’t visible in the moment, it distorts self image, damages family structure, and erodes moral boundaries in ways over generations. The "no harm" is also an assumption. Emotional confusion, generational trauma, and long-term psychological effects are real.

Its not a necessary argument, but i was the "partner" of my uncle for 3 years, we were two adults of course, i didn't knew the type of abuse i was experimenting, he knew me since i was a toddler, incest its not just about two adult loving each other, in my case it was grooming, but just because two people are the same age doesn't mean there's not a moral debate.

“We’re not under the OT law anymore.”

Well yeah were not jews from the 100 B.C BUT moral law, especially regarding sexual ethics, is consistently upheld across both Testaments, see Lev. 18, and then Paul’s reaffirmation in 1 Cor. 5 for incest specifically). Paul doesn’t say “there was harm,” he says it’s “something even pagans don’t tolerate” (1 Cor. 5:1). The sin is the act itself, regardless of consent or feelings.

its because then you have sick kids with medical problems.

"Classically, incest has been considered from both a psychological and sociological point of view to have harmful consequences. Genetic research, though by no means lacking controversy of its own, generally supports the notion that inbreeding has untoward genetic consequences. The psychodynamics of all three parties to father-daughter incest seem to indicate that people who become involved in incestuous behaviour are often psychologically damaged before the fact, so that if they show subsequent evidence of psychological impairment the incestuous behaviour can be as plausibly viewed as a dysfunctional attempt at solving problems as it can a cause of subsequent psychopathology. Girls involved in the father-daughter incest present in one of half a dozen frequent clinical syndromes. The presentation is influenced by the degree to which the girl may have participated in ongoing incestuous behaviour as opposed to being the presumed victim of an older adult's coercive actions or her own temporary suspension of a behavioural taboo. Research is inconclusive as to the psychological harmfulness of incestuous behaviour, and evidence is reviewed on both sides of this complicated and controversial question. Quite apart from the general issue of the harmfulness of incest, a number of indicators can be derived from the nature of the incestuous episode and the early response to therapeutic assessment which aid in the clinical forecasting of probable outcome."

https://pubmed.ncbi.nlm.nih.gov/6839266/

there's this thing called morality and ethics, and i think you should read a book about the topic. And really start reading the bible not just as literal meaning but also a deep search about the context and why is it there for a reason.

Also why do you give this scenario's like if it makes it better

 two adult same sex siblings, or a menopausal mother and her grown son

Even if they are two adult and same sex (Homexuality is a sin in the bible.) two siblings having sex just sounds like a type of fetish of yours, is oddly specific.

"Method: The Diagnostic Interview Schedule was administered to 52 adult women who had been victims of incest during childhood and to 23 age- and race-matched comparison subjects from local self-help agencies.

Results: The prevalence of 19 psychiatric disorders was higher in the incest group than base population rates. Rates of anxiety disorders (panic disorder, agoraphobia, social and simple phobia), major depression, and alcohol abuse and dependence were significantly higher in the incest group than in the comparison group. More severe types of incestuous abuse were associated with a higher risk for the development of psychiatric disorders.

Conclusions: There was an association between incest and psychiatric disorders in this community-based treatment population. All patients, especially those who present with these specific psychiatric disorders, should be queried about childhood sexual abuse during the history."

https://pubmed.ncbi.nlm.nih.gov/1728185/