Board Thread:Questions and Answers/@comment-26457350-20150630163548/@comment-5774380-20150630164506

They are part of the same bloodline. Read this: Ackerman family.

The next part of my answer is spoiler for anyone that is not up to date with the manga:

Kenny implied that Mikasa's father is part of his close family, and due to Kenny being Levi's uncle that would mean that Mikasa and Levi are in fact somewhat close related.