Same-sex unions are recorded in the history of a number of cultures, but marriage or similarly formalized same-sex unions were rare or nonexistent in other cultures.
Using this measure, siblings are actually 99.95% the same. The 50% refers more to the amount of DNA we get from mom and the amount we get from dad. So we are more similar to our relatives than we are to a mouse. Let's explore in a bit more detail what the 99.95% and the 50% numbers mean.
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%.