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.
A brother to sister (sibling to sibling DNA Test) works by testing a very large number of different DNA points (known as loci) in each person. Full siblings share a lot more DNA in common than do unrelated persons, and also, half siblings share more DNA in common than unrelated persons, but not as much as full siblings. So this test looks at the amount of DNA sharing and uses DNA statistics to evaluate the amount of DNA sharing to deduce biological relationship.