The theories on this one are blowing up the Internet. I read about one today that makes my theory look like a joke. I believe both Ray and Ben are Luke's kids raised by other people just like Luke and Leia were. Leia took Ben, her nephew and raised him as her own just like she was raised, Ray was raised by a rebel family just like Luke was but backwards in a way. The newest theory I heard was that the Skywalker in the title is referring to a group of people who are strong with the force but never became Jedi's who are called Skywalkers, they were used to navigate through space thanks to their powers thus why they are called Skywalkers. This comes from one of the many books that are part of the Star Wars universe. I may not have the theory explained correctly here but its more or less what I remember and it sounds amazing.
