Considerate literally comes from considering (aka thinking of other people when making decisions), if you forget to take other people into account when making decisions, you’re being inconsiderate.

It’s not complicated.

Imagine caring as a value, for example, my care™ about this topic is 2, whereas yours is 5.

In this interpretation, “I don’t care” implies that the care™ value of the speaker is 0, by the same logic, saying “I care” implies that their care™ value is greater than 0.

With that in mind, “I could care less” implies there is a care™ value lower than the one they currently hold. Meanwhile, “I couldn’t care less” implies the opposite, there is no care™ value lower, which is only true for 0 (AKA “I don’t care”).

Tbf, incest is quite often the result of grooming, which brings us back to the consent issue.