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 Stan and Pierce do not have the same copies of each of the markers, they both always have one marker from their mom and one marker from their dad. Which means they are almost certainly brothers. So there you have it. With simple DNA tests, siblings can match at any number of markers. They can match at all, none, or somewhere in between. This has nothing to do with how related they actually are at the DNA level. Siblings are about 50% related.