That gets reported by anime sites because of the incest issues, the fact they lived in different parts of the world before coming to promise island and because later releases of the game spinoffs may have said adopted after the Tokyo laws about blood related relationships in art went into effect. (However, originally, the creators have said they were blood related siblings in both the games and anime from everything I've heard or read.) Also, remember this did not start out as an eroge game, as it was originally a playstation 1 game, so sex scenes and incest wasn't an issue. I have no idea if it ever was turned into an Eroge at any point, however if it was that could also be why, though I don't think it was.
Though, I've read that originally the game only had 8 girls, 4 were added specifically for the anime version and then added to the game at a later time in a second edition version. One of which is my favorite character, Aria.
Do NOT read this spoiler passage unless you want the thoughts contained within to creep you out a bit. You have been warned. It is just my general understanding of the reasoning behind the series and should not be taken as fact, but it is very creepy.