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.
Sibling DNA testing http://www.dnatesting.com/dna-paternity-test/more-dna-testing/sibling-dna-test determines if two individuals share one or both parents. DNA Testing with a possible brother or sister can help answer paternity questions when the possible father is not available to be tested. Half siblings share one parent. For example, a brother and a sister may share the same mother, but not the same father.