On the contrary, Cindy McCain
does go out and stump for John McCain. Here's just one example (
Cindy McCain to Visit Tallahassee, Tampa Bay). It doesn't get the kind of press that Michelle Obama gets because Michelle is a much more dynamic and interesting figure to watch. Cindy McCain, like Laura Bush, is a sweetheart, and, therefore, not particularly exciting for the press.
As far as brothers go, I would argue that it's not
as pathetic as going after someone's wife, but it's not much better. The entire point is that these people are not necessarily making conscious choices to go out there and stump for their loved ones, as much as there is an unspoken obligation to support your spouse, children, etc.
And I think Obama's issue isn't even necessarily that people are
speaking about his wife. If people were constantly talking about how gracious she is, I'm sure it wouldn't bother him. It's the idea that people are saying horribly untrue and malicious things about her. They paint her as some kind of Assata Shakur when anyone who listens to her knows she is nothing like that. There's an underlying racist theme that is constantly used to drive fear into people who aren't particularly comfortable around black folks. They're saying, "Do you really want this black nationalist in the White House?"
Obama's way around it is to suggest that they attack him rather than his wife. I don't see anything wrong with that.