And it must also be asked why two family members would even want to bother trying to be married, when most of the benefits of marriage are directly related to making a person another loves, who is not already a part of the family, a part of the family. I'm not exactly sure why some of those other benefits are given to spouses but not immediate blood relatives.
Some of them are given to immediate blood relations though with certain circumstances. For instance, a single person can claim head-of-household on their taxes with a qualifying dependent that is not their spouse or child. I know this because I did it the year before I got married, with my sister. She lived with me for the year, I paid all the bills, and she made below the limit for me to claim her on my taxes. I was able to claim head-of-household and get an extra tax deduction. I believe that it is to encourage people to actually take in their relatives that can't work or can't find a job or who might be going to school. This exemption is only available for blood relations, in-laws, step-family, or foster/adopted children. It is not available for a significant other who isn't a federally recognized spouse.