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.
Since mom's two X chromosomes can recombine, her sons and daughters will each get a mixed X from her. A brother and sister, then, would have about 50% of their maternal X DNA in common. The same is true for sisters. But sisters have another whole X from dad that's almost exactly the same. Of course, this is just one chromosome that is only about 5% of the total amount of DNA.