Capitals are funny sometimes because we have weird rules for them. But as far as I know in this case brother and mother should be lowercase. You do have to remember that mistakes are made by writers and editors all the time, so don't think that just because it's in a novel it's perfect English.
The reason I don't believe they should be capitalized is simply because they are common nouns. If we used the brother's name, that would be a proper noun and should be capitalized.