Hi! we usually hear the brothers share 50% DNA, and cousins share 12.5% DNA etc.
is it accurate?
is it possible for brothers not to share any DNA sequence at all?
I mean, theoretically, if both receive exactly chromosomes from the "other source", one brother from the father, and the other from the mother, for each chronmosom.
so, the question is, if it's possible theoretically for brothers not to share DNA, and a second question: is it right to conclude that on avarage, they DO share 50%?

You're thinking is absolutely correct, it is merely probability that means we are likely to share 50% of our genes with our brothers and sisters, and can and probably does vary to some extent. But as we cannot tell whether or not we share more genes with one sibling or another (and in all likelihood it will be 50% or thereabouts so it makes no real difference), then behavioural ecology demonstrates that animals and humans treat all siblings equally.
See another set of answers that complements your question as well
http://www.askabiologist.org.uk/answers … hp?id=5746

One final point, it is mentioned in the other set of answers as well, but there is a process called crossing over, it happens every time a sperm or egg is made, whereby paired chromosomes shuffle their genes between each other, it generates more genetic variation, and prevents a situation where you could inherit the 'other' chromosome to the one your sibling got, by shuffling the deck as it were, genes do not get inherited in as large a set of clumps and means that the while the average relatedness between siblings is 50%, it will also mean that the amount that figure can vary will be smaller as well.

To add on to what has already been said,
There is some DNA that brothers will always have in common. One is the DNA of mitochondria, essential cellular organelles that are passed from the mother. Additionally, brothers will both have a Y chromosome from their father in common (their mother will be XX, and since they're male, they must be XY.) Since mitochondrial and Y chromosome inheritance are direct-line maternal and paternal, respectively, they are useful for studying ancestry.