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.
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.