Board Thread:Questions and Answers/@comment-26457350-20150630163548/@comment-5774380-20150630164535

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 close related.