Is " extrovert-introvert " siblings a myth?

We read stories which portray diametrically opposite siblings. Is it a cliché which was invented to base a juicy story? Studies have confirmed that this is the norm.
The phenomenon is called sibling de-identification, popularized by American psychologist Feinberg, he says this occurs predominantly in siblings of same sex, have limited age differences and those who are immediate ones (the first and third sibling tend to be similar than first and second, say). The second one tries to pursue different courses, enjoys different genre of movies, have different hobbies, which is acceptable. But what is surprising is that they have a very different personality traits even though brought in exactly the same environment- the second one is reserved if the first is enterprising, short tempered if first is warm, takes quick decisions if first is contemplative etc. Research shows the second girl of the family tends to be tomboyish and lacks feminine interests like dance, reading romantic novels .....
Why does this happen? Siblings compete for parents’ share of attention and love all the time, which leads to friction (prominent cause of sibling rivalry). Over time, they (mainly the second one) try to differentiate themselves from the each other. (In marketing terms, they tend to carve out and operate in separate niches so that they don’t have to compete for the customer (in this case the parents) directly.) So, by having different personality, they unconsciously establish they are unique in their parents’ eyes.

We are all victims of sibling-de-identification at some point in time ; arent we ???