Take a look at the shared DNA between my father and his third cousin, who was known before the DNA studies. (Third cousins share a pair of great great grandparents.)
My mother has several matches (excluding her brother and me) who share more genetic material than this. Most do not respond to requests to exchange information, which is very disappointing. The select few who have shared information with me can now be compared to one another. The idea is that genetic cousins can be clustered together based on matching one another, forming a working family tree. When I compared my mother's closest relations to one another, I was surprised to see quite a few of them matched each other even more closely. The problem is that nobody knows how anybody else is related. In theory, if we trace back to great great grandparents, we would be looking at identical trees in one branch. This has not happened yet.