Same-sex marriage, also known as gay marriage, is marriage between people of the same sex, either as a secular civil ceremony or in a religious setting. 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.
DNA between siblings are similar if you share the same biological parents. The genetic information you gain comes from both of your parents, and are a result of fertilization of 2 haploid cells fusing to form a zygote that has a diploid number of chromosomes.
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.