User blog comment:Miskos3/Chapter 56, Review/@comment-24930713-20140511220212/@comment-24930713-20140513022847

I doubt he'd tell us something like that, that would take all the mystery out of it. And considering that titans have been around for about 100 years and eaten most of humanity. It's likely that there are gonna be people left with the same last name who aren't related to each other. Not to imply that I don't think it's a little weird that Mikasa and Kaney have the same last name.(not including Levi since if I'm understanding right, he didn't have that last name from birth) But I don't know... I don't feel like it's something that would be important to the story.

Although I did see someone's thoery on tumblr about maybe it being a cult type thing. But I guess we'll just have to wait and see. :(