Alongside the allosomes (X and Y chromosomes), the autosomes make up the other twenty-two chromosome pairs in every cell in the human body. You inherit exactly 50% of your autosomal DNA from your mother and exactly 50% of your autosomal DNA from your father. Strangely, however, you do not inherit exactly 25% of your autosomal DNA from each of your four grandparents.
In each pair of autosomes, one comes from your mother and one comes from your father. They, in turn, got one of each of their pairs from their mother and the other from their father, but before passing the autosomes on, they are randomly recombined. The further back you go, the smaller the percentage of autosomal DNA you will have inherited from each individual in that generation. Because of the chance involved in determining which copy is inherited at each generation the amount of DNA you inherit from your different ancestors is very variable.
In the same way that you do not inherit exactly 25% of your autosomal DNA from your grandparents, you do not share exactly 50% of your autosomal DNA with siblings. This generally ranges from 38% to 62% and means that you can be genetically closer to one sibling than to another.