There's a game I like to play with my friends sometimes called "Watch This!" Whenever anybody's wondering about some obscure fact (for instance: "Who was Saint Louis?") I will say, "Watch this!" then pick up the nearest phone and dial my sister's number. Sometimes I'll catch her in the car, driving her kids home from school in the Volvo, and she will muse: "Saint Louis . . . well, he was a hairshirt-wearing French king, actually, which is interesting because . . ."

So my sister comes to visit me in Rome—in my new city—and then shows it to me. This is Rome, Catherine-style. Full of facts and dates and architecture that I do not see because my mind does not work in that way. The only thing I ever want to know about any place or any person is the story, this is the only thing I watch for—never for aesthetic details. (Sofie came to my apartment a month after I'd moved into the place and said, "Nice pink bathroom," and this was the first time I'd noticed that it was, indeed, pink. Bright pink, from floor to ceiling, bright pink tile everywhere—I honestly hadn't seen it before.) But my sister's trained eye picks up the Gothic, or Romanesque, or Byzantine features of a building, the pattern of the church floor, or the dim sketch of the unfinished fresco hidden behind the altar. She strides across Rome on her long legs (we used to call her "Catherine-of-the-Three-Foot-Long-Femurs") and I hasten after her, as I have since toddlerhood, taking two eager steps to her every one.

"See, Liz?" she says, "See how they just slapped that nineteenth-century façade over that brickwork? I bet if we turn the corner we'll find . . . yes! . . . see, they did use the original Roman monoliths as supporting beams, probably because they didn't have the manpower to move them . . . yes, I quite like the jumble-sale quality of this basilica. . ."

Catherine carries the map and her Michelin Green Guide, and I carry our picnic lunch (two of those big softball-sized rolls of bread, spicy sausage, pickled sardines wrapped around meaty green olives, a mushroom pâté that tastes like a forest, balls of smoked mozzarella, peppered and grilled arugula, cherry tomatoes, pecorino cheese, mineral water and a split of cold white wine), and while I wonder when we're going to eat, she wonders aloud, "Why don't people talk more about the Council of Trent?"

She takes me into dozens of churches in Rome, and I can't keep them straight—St. This and St. That, and St. Somebody of the Barefoot Penitents of Righteous Misery . . . but just because I cannot remember the names or details of all these buttresses and cornices is not to say that I do not love to be inside these places with my sister, whose cobalt eyes miss nothing. I don't remember the name of the church that had those frescoes that looked so much like American WPA New Deal heroic murals, but I do remember Catherine pointing them out to me and saying, "You gotta love those Franklin Roosevelt popes up there . . ." I also remember the morning we woke early and went to mass at St. Susanna, and held each other's hands as we listened to the nuns there chanting their daybreak Gregorian hymns, both of us in tears from the echoing haunt of their prayers. My sister is not a religious person. Nobody in my family really is. (I've taken to calling myself the "white sheep" of the family.) My spiritual investigations interest my sister mostly from a point of intellectual curiosity. "I think that kind of faith is so beautiful," she whispers to me in the church, "but I can't do it, I just can't . . ."