When you are part of a culture, you take things for granted. There are concepts and associations that don't get mentioned for two reasons: first, that they are so commonplace that you don't think of them; and second that the concept is often amorphous and hard to describe.
In the American Culture I have lived in, since I was too young to really understand the full meaning of the term, brother-in-laws have been understood to be sketchy, unreliable, and slightly criminal. Many people who have never had a negative experience with a brother-in-law, or who may even be a fine, upstanding person who is a brother-in-law themselves, would probably understand the usage of "brother-in-law" as being a shorthand for a kooky, moochie, ne'er-do-well. Somewhere in some sociology department there is probably a professor pouring over Spanish War era letters and diaries, trying to find the locus classicus of the besmirchment of brother-in-laws, and someday we may be able to understand exactly how this association came about. But I will just establish that such an association does exist, and will even go so far as to ask my reader what inference they would make if a friend of theirs said "I loaned my brother-in-law $500 for car repairs".
Part of the reason that brother-in-laws are seen as perhaps dangerous interlopers lies in the fracture lines that can be seen between the nuclear family and extended family in American culture. While American culture's dependence on the nuclear over the extended family is perhaps overemphasized, it is true that while most Americans keep in some sort of contact with their extended family, they are only intimate with their nuclear family. As an American, you are expected to associate with members of your extended family, but they are often not held to the same standards of behavior, and can not be openly reprimanded, in the way that your nuclear family can. So, with a brother-in-law (or a similiar relative, such as a cousin, uncle, step-sibling, half-sibling, or any combination thereof), you would be expected to support them in a number of ways, while not having any real emotional connection or control over their behavior. Such a situation is a recipe for frustration.
This impression, and the analysis of its causes, comes from the fact that I am a member of the American culture, which means I am both expert and totally unreliable to comment on this matter. I would be interested in hearing the viewpoints and impressions of other people in my culture, as well as other people in both Anglophone and other foreign cultures.