Looking at it from a pragmatic perspective, I see 2 reasons why incest was historically frowned upon.

The first is the risk of genetic diseases, decrease in immune system performance, the likes. Children from incestuous relationships have worse genes, so the easiest way to prevent that was to prevent incestuous relationships to begin with.
Now ofc, we have contraceptives, abortions, hell soon we will have gene editing of both embryos and full humans, meaning there are plenty enough other ways to stop inbred children. From that perspective I see no reason to have any specific laws at all, relationships should be legally treated all the same regardless of the genetic overlap of the participants. A ban on knowingly making and carrying inbred children would fulfil the same use and likely much more effectively.

The second issue I see would be that strong bonds like family make it easier to be forced into a relationship. Authority can and does get abused to force sexual relationships, think of the Hollywood scandals to that length for just one example. You sometimes see companies banning relationships with someone’s superiors for example, under the assumption that it is just too likely such a relationship isn’t entirely for immaterial reasons (both down and up, though in the case of incest only down (think parent forcing their child to have sex or get no college money) would be an issue). Historically, given there where already other issues with incest (inbreeding), the interest for a proper detailed handling of those potential issues was low, a blanket ban was seen as acceptable. Given that the other issue is now no longer a problem, I think it is time to address this properly and look into how much regulation is necessary to achieve a reasonably low rate of those abuses occurring.
Seeing that the law doesn’t generally care too much about positions of authority across sexual relationships, and the cases where it does are usually relatively vague (abusing a position as the “breadwinner” should be covered no matter if as a boss or a parent in countries that do cover it), I think you could get away with largely deregulating it and adding laws later as needed. Whether or not you should still keep direct parent-child relationships illegal initially I am not sure, this is very much out of my depth.