Facebook could be smarter

For a social networking application, Facebook does a poor job of understanding my network. For example, why doesn't Facebook use its existing knowledge of relationships to make it easier to confirm new relationships?

My wife had already confirmed that her sister was her sister and that I was her husband so why didn't Facebook know that my sister-in-law was my sister-in-law when I added her as a friend? The process to add her and explain our relationship could be significantly streamlined and Facebook could be smarter.