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.
Identical twins have exactly the same DNA. They will be the same sex and they are more likely to look like each other. Fraternal or non-identical twins develop from two separate fertilized eggs. Non-identical twins will share on average half of their DNA, similar to the percentage of DNA shared by two siblings who are not twins. Fraternal twins can be the same or different genders.