Identical twin brothers share the same DNA; other brothers do not. Since children get half of their DNA from one parent and half from the other, siblings with the same parents typically share about half of their DNA.
Siblings who share only one parent have approximately one quarter of their DNA in common. However, DNA does not always copy perfectly, and small errors are common. Small mutations and recombination errors ensure that even twins won't share exactly the same DNA. These small errors can cause health problems, but they are also a main driver of evolution, and humans wouldn't have evolved without them.