I have blessedly enough never encountered anime or manga with parent-child incest (other than in the form of clear domestic violence). The regular sibling one was tough enough for me to get used to, and it still bothers me every now and then, especially when there would be better partners around (like Kuroneko in Oreimo, to give an extreme example, although I never finished that show after hearing how it approx ends).

After all these years I think I'd enjoy a show where a brother and sister would be really close and friendly and would even purposefully have fun by tricking their acquaintances by hinting there could be more (like that one scene in Mahouka superficially), but in reality neither would be in love with the other in a man-woman way.