fraternal love

I've always wondered what is it that the Japanese mangakas and animators like so much about siblings incest. Is it much more tolerated than in western cultures? (of course my question concerns the realm of shoujo and romance stories, i mean i wouldn't find it as surprising if the anime in question would be hentai or a similar genre)
The general idea is that romance and sexual relationships between brother and sister is wrong in so many levels. Yet again when it comes to manga and anime the situation seem to be different. The protagonists are never condemned for their behaviour. Most of the times instead they are depicted as star crossed lovers whose sympathy is well deserved.
The first manga i ever came across was Lady Georgie. Technically I can't say it was incest as the protagonists weren't blood related but since they were raised as siblings for so many years it kinda shocked me that both Abel and Arthur were able to develop such feelings for their little sister. Not to mention Georgie herself who is the last one to know the truth about their "brothers"
Then Marmalade boy came along. It has to be said that in this story there are more extenuating circumstances. First Miki and Yuu don't grow up as brother and sister. At 15 years of age they suddenly become step siblings because of their family remarriages and fall in love virtually from day one. Second, they discover they may be blood related siblings after they have fallen for eachother but decide to be together regardless. Third, They aren't blood related after all ( lol)
More recently there is Vampire Knight. Yuki and Kaname'are brother and sister ( although we all know that Kaname is not the "real" Kaname but the reincarnation of the Kurans ancestor..I'm digressing here) but they belong to a very high elite of pureblood vampires hence they are allowed to marry and love each other ( their parents in fact were brother and sister...). Anyway even on this occasion I am able to overlook the incest factor because the story is not real life based. Besides vampires are known for more dreadful actions ( sucking other people's blood that is) so incest doesn't seem to be the worst connotation.

In "I love my little sister" however, the situation is different. The twin brothers Iku and Yori don't have any plausible excuse. The love story was presented in a very sweet manner but yet I couldn't root for them .No matter all the fluff, tears and romance. l i couldn't get past the fact that those two making out shared the same womb for nine months. To me, rather than their hearts it was their hormones talking..