Unfortunately it is unsolved mystery,those are the only 2 times his brother is mentioned and there is much more of him claiming otherwise.
- It could only be a common "movie" mistake,since there is not any weight put on his brother he is only used as a subject in a response.
- In other case it could only be that his is just sarcastic when telling he is only child,like he has a grunge against his brother.
I would go with a "movie" mistake however.