Siblings are different from each other because of how sexual reproduction works. People's cells have chromosomes that contain their DNA, and these chromosomes are organized into 23 pairs. When people reproduce, the father contributes a copy of each chromosome for the child and the mother contributes a copy of each as well. Which gene is passed on to the child -- the mother's or the father's -- is completely random. This leads to massive variation, which is why two brothers or sisters, even fraternal twins, can have different hair color or blood types.
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