@Thrembs
I fairly positive this ends with 0 incest. Pretty sure this is just her reflecting on what once was a potential romantic interest in her sister, but never saw fruit. So she is starting to move on more or less, likely towards the other girl, but likely only after a few chapters.
Also, iirc, it is legal to marry your cousin in Japan, and the story is only featuring a 1 sided interest between 2 sisters, so no genetics will get pass on to their impossible children. Also, not to discredit or downplay your experience with inbreeding/incest, but I dont think the same necessarily applies here. This is a 1-off at best. Not a cultural tradition going on for generations. Genetic issues are very improbable for a 1st generation child of inbreeding. And while that is not to say it wont ever happen on the 1st generation, it usually takes a few generations to notice any kind of significant genetic weakness. Sure there could always be one who just happened to hit the genetic disorder jackpot, but overall there is not too much risk in inbreeding for the first few generations, and even then it is only like a 3-4% chance, and 1-2% or so is about average for the non-inbred population. Plus, as I mentioned earlier, they are both female, so there is literally 0 chance for them to have genetically related children with each other.
I dont think they are idealizing it by any means. Sure such manga exist, but this is not likely to be one of them. But whether you continue or drop is ultimately up to you in the end.