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.
Here is my full sister and how much DNA I share with her as well as the number of segments that we have in common. Siblings are expected to share about 50% of their DNA --- my sister and I share slightly more than expected, just by random chance. Getting your siblings tested at 23andme is very useful because they will have DNA (and relatives along that DNA) that you do not have.