While our children share genetics it rarely means they share similar personalities and most siblings couldn't be more dissimilar.

Did you ever wonder why you and your Sister or Brother are poles apart?
You may look alike but you have completely different personalities and characteristics? Now that I have kids myself I often wonder how (having raised my tots exactly the same) they are so unalike. Yeah, they look kind of similar and because of their flame red hair it is fairly obvious they are related but as for their temperaments- well they fight a lot for a reason if you know what I mean.
While our children share genetics it rarely means they share similar personalities and most siblings couldn't be more dissimilar.
It beggars belief that children that are raised in the same way with the very same upbringing can be so utterly diverse. However we as parents are looking at our children in a one dimensional way, forgetting that there are other environments (school) to consider that carve out their character. Home experiences are akin to our siblings but school experiences are not the same for each of your kids and friends that our nippers befriend can alter them too.
Theories
There are theories as to why siblings are different and one of them is that a child will specialise in something not identical to their sibling. For instance if the eldest sibling is an academic and does brilliantly at school, the other kid will try find their niche and consciously or subconsciously strive to be different.
Another theory is that while all siblings share the same home environment that they were different ages when experiencing life events that can change a child’s personality. For instance a death of a loved one or a parent’s separation; an older child could have understood more and felt more upheaval to that life changing event.
The last theory is that parents are to blame as we label our children whether rightly or wrongly, who doesn't say “this is Lucy she is top of her class” or “this is James and he is a top footballer”. This can influence a sibling’s choice of what they would like to be or what activities they want to partake in. However this can damage a kid’s self-esteem if they hear that they are less gifted then a sibling and every child should have a chance to develop to their full potential. 
There is a chance that sibling DE-identification occurs which is when a sibling actively tries to be different to their brother or sister. It can cause conflict when two or more siblings are alike so to avoid sibling rivalry a kid will endeavour to achieve merits not comparable to their sisters or brothers. Siblings basically want their own identity and whether they realise it or not they are alleviating future rivalries between siblings by seeking their own path and finding their own place in the world.
So next time you start to wonder why your cherub is quiet and introverted while their sibling is as mad as a cracked pot, consider that a label sticks and have a little think that each one of your brood is an individual who may be cut from the same cloth but that doesn't mean they are the same!