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.
Each sibling gets the same distribution of parental DNA in terms of percentages. That is, each sibling gets half of their DNA from the mother and half from the father*. However, each sibling gets different parts of the two parents' DNA. It always adds up to 50:50, but if you have a gene where the father has the two versions A and a and the mother has the two versions B and b, you could have siblings that end up with AB, Ab, aB, or ab.