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.
Even though identical twins have the exact same DNA, though, they aren't really perfectly identical. That's why every mother can tell her identical twin babies apart. The reason they aren't identical is that each of them develops separately, and lives a separate life.