The brother-sister relationship is the most understudied in the whole neurotic caboodle of American family ties: Mother-daughter and sister-sister bonds claim far more time on the nation’s talk shows and psychiatric couches. But the fraternal lens brings more than one First Lady into sharper focus.

Think for a moment of the lumpy, shady Rodham brothers, Tony and Hugh, not doing any worse with the paternal martinet than little overachieving, I’ll-show-’em Hillary. It’s also no surprise that having a younger brother, Hall, only imposed one more social-working burden upon Eleanor Roosevelt, who watched him subside into alcoholism before she gave him a White House funeral. Laura Bush? A classic, bookish only child—but perhaps one who did some unconscious searching for a wiseacre brother she could straighten out and be continually exasperated by.

It’s easier to picture tall, muscled Michelle as an older sister than a younger one; you see her marching down to the playground on a little bro’s behalf, hotly pursuing the scruff of some bully’s neck. But as basketball star Craig Robinson’s little sister, the baby of the family, she took her first steps inside an especially secure perimeter. When he strode out in front of the Democratic convention to introduce her, one realized: This explains a lot. Someone has had her back for her entire life.

Related:

• Archive: “Cover Story”
• Table of Contents: Mar 23, 2009 issue of New York | Subscribe!