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.
Because you and your sister are identical twins, you pretty much have the same DNA. Not exactly but pretty darned close. This is because you both started from the same fertilized egg. Which is why identical twins are also called monozygotic. Since you both have the same DNA, this is almost like the two sets of children have the same moms but different dads. In fact, this is why at the DNA level they are really more like half siblings instead of first cousins.