Check The Source?

Hey, just had a peculiar incident today. I was having a conversation with a friend when the subject came to a story about a riot or something of the like. My friend said that he knows how exactly the event happened because of a family member that was there. However, his version of the story did not match up with the police report that he told me was written out. To sum it up, the police report did not match the family member's report, and most likely, the report was written by the police was written by an office clerk or something. My friend later said that the only reason no one takes his version of the story seriously, is because he heard it from someone related to him.

My question is: Does being related to a witness who passes the story to you make the story less plausible than someone who wrote it from an authority figure, like a police officer? People generally tend to disbelieve something that someone heard from a family member because the familial relation might have skewed the truth, or so I have heard.