Well, cultures are different, so what is acceptible to be aired on tv is different, just look at the amount of nudity, violence, language that can be aired on tv in Japan with anime. As for the incest, its just one of the many themes that can show up in anime/manga. From the two examples shown, Lefy said it was comedic purpose for Onegai, so can see why it was used and why it showed up on TV, and as for Angel Sanctuary, it wasn't shown on TV, it was an OVA, so people would of had to buy it directly to watch it. I know campus cumbustible guardian also seems to deal with this too, but that one is also an OVA..and as we know OVAs are less strict than TV.