[Even Mal could tell he had made some sort of error, but he couldn't take it back. He couldn't explain the feeling it gave him to see that thing. The reasons why were his own, and he wasn't ready to divulge them. He might never be. He could see that Alex was sad now, but he wasn't even exactly sure why. All he could do was push on.] You have a sibling. But you don't know who or where they are. [Just trying to keep up with the facts, at this point.
His expression did change then. He didn't much think about it, but he reached out and put his left hand on his shoulder.] If that's true, you owe him nothing. [He met his gaze. He was completely confident that what he said was true.] If I was you, I would kill him.