If I get the question right, the answer is 6.25%, not 25% as the other answer says. Its easy to calculate. Siblings share 50% of DNA on average, it also doesn't deviate much from it as the shared HIR (half identical regions) are very long and can sometimes cover the whole chromosome. There are also the least amount of recombinations within one generation. Half siblings have half of that, as the name says, so 25%.