Why is that?

Siblings don’t have the same DNA. It is primarily a mixture of the genomes of the biological father and the biological mother. This statement would be quite correct for “one” twins.

Here (e. twins) there are roughly two possibilities for development. Since the behaviour of the environment in each twin is almost identical and they also feel a connection in this way they develop similar. To what extent character properties play through genetics is controversial. I see this triggered primarily by externalities (free to Freud: The Over-I develops analogue).

However, there is also the possibility that a twin deliberately separates, so behaves deliberately differently than the other in order not to be seen more than “copy”. Not necessarily by disintegration but by the desire for uniqueness. This changed behavior (primarily conscious) leads to other choices, other friends to another environment and ultimately to another development of character.

Normal siblings (even with the same sex) develop particularly frequently. You are likely to have similar problems triggering different feelings (injustice=>Wut, insecure family conditions possibly => Angst, ..) It is possible to deal with different ways: living out, controlling or unconsciously (the last person does not feel this feeling consciously). Therefore, siblings usually have the same negative feeling as the main problem in childhood, but in the course of demarcation they develop different “survival mechanisms”. For example, 1 child full of inner fears but constantly strives outwards not to show that and also do not let his own life influence too much (feeling = bad), a 2nd child that is fully alive, constantly shows his fear. And a third that apparently does not know fear (highest fear) from boredom. The latter will easily get into danger, but usually come without prejudice to it, as they do not know “bade fear”, even from potential threats by other people. But not always – drug experiences are almost to be expected.