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.
People are usually surprised to find that, when it comes to our genes, we are just as related to our parents as we are to our siblings! We share 1/2 of our genetic material with our mother and 1/2 with our father. We also share 1/2 of our DNA, on average, with our brothers and sisters. Identical twins are an exception to this rule. They share their entire DNA between them.