Those, it turns out, are the easier questions to answer. The difference between the two sisters lies in one word: schizophrenia.
After she turned 20, Catherine had no job, no children, no holidays. She had been sectioned in more than one psychiatric hospital and briefly imprisoned twice. She lived, without any lovers, in an impossibly untidy council flat in Bristol in which she had hardly any visitors and certainly no visits from her own family. Whenever any of them called on her, she shut the door in their faces.
Her sister, the elegant 39-year-old across the table from me in her Oxford club, and the author of one of the most moving and compelling memoirs of the year, could hardly be more different. Bright, confident and, she admits herself, "driven", Mary Loudon is the kind of person who sets targets. In her early twenties she determined that by the age of 30 she would be earning a living from her writing. When she was 30 she resolved that within a year she would meet the man she would marry.
That's the way it happened. The loving husband with a high-powered job (and a brain "like Jonathan Miller"), then two children, with a third on the way in June. The trail of admiring reviews (I wrote one myself) for her three nonfiction books. The second house in the Wye Valley, the two skiing holidays a year in Courcheval, the A-list friends: all the good things in life seemed to be unrolling smoothly ahead of her.
When Catherine died, Loudon was plunged into something deeper than grief. She'd only been nine when her sister, 13 years older, had walked out of her family's life for good: there'd only been a few letters from her since, and hardly any visits. She really was, as in the title of her book, a relative stranger.
Childhood memories of Catherine would bubble up, pin-sharp with loss, but they didn't tell her what she wanted to know. How had she lived? What had she done with her life? Had she any friends? Had she known happiness? Who had cared for her, looked after her, helped her? Had anyone?
As she sat in the Bristol hospital morgue next to her sister's covered body, Loudon certainly wasn't thinking of writing a book about her. There was, she says, no particular moment when she realised that she would, no sudden decision to go on a quest that would take her back, through other people's memories, into the life of the sister she'd loved as a child but never completely known. It's just the way it happened. On the oncology ward where Catherine had died, she made her first discovery. The nurses there knew her as Stevie. They knew she wasn't a man, but that she wanted to be treated as one. Mercifully, that's what they did. Even when she visited her dead sister's dark, decaying flat, with its piles of junk and discarded clothes up to 5ft deep, even when she took pictures of the nightmarish paintings and the senseless, scrawled slogans on the walls and felt the beating of the wings of madness, even then the thought of writing a book still had not crossed her mind.
"All I was wanting to do was preserve something of my sister," she says. "This was the first and only time I'd been in a home she'd lived in since I was 24 - ten whole years. You don't go about photographing someone's room in minute detail if you see them a couple of times a year and you've got the odd snap of them on your mantelpiece. But this was all I had of Catherine.
"It's the same with seeing her body. For once, I didn't need her permission to go and see her. I don't think that she didn't want to see us when she was dying, it's just that she couldn't have handled it. She couldn't handle the whole idea of family. That's quite common among schizophrenics.
"All the relationships she had with people were quite minimal. She didn't have people into her house, didn't have lovers, didn't have best friends, didn't eat with people and talk to them the way that we do. Within that context, not being able to handle the people with whom she once lived most closely was not so surprising."
To find out more about Catherine's last years Loudon had to talk to strangers. And it's here that this book, which from the start has been direct and purposeful, becomes absolutely fascinating.
As her three previous books - about nuns, the clergy, and the town she grew up in - have shown, Loudon is an interviewer with an almost forensic empathy. She works on a far smaller scale than the great American interviewer Studs Terkel, but her books are his nearest British equivalent in that they, too, dig deeper into ordinary people's lives and, in his phrase, "celebrate the uncelebrated".
The commonest objection to books such as these is that a collection of interviews with ordinary people is too disparate, too unpredictable, to tell a straightforward story. But with Relative Stranger that doesn't apply. At its heart it is that most compelling of narrative forms, a quest - and a resurrection quest at that.
And as Loudon tries to find out more about her sister we learn more about a side of Britain that never makes the headlines. Who cared for one forgotten crop-haired schizophrenic woman who called herself Steve, was convinced she had been tortured by Tibetan monks, and would occasionally go into churches and shout at the minister while he was giving a sermon? Whose life did she brighten? Who saw through the mental illness to the woman?
A few did. Sister Paul, an 80-year-old Catholic nun, for one: she would look forward to seeing Catherine on her daily loping walk by the convent. Catherine, she told Loudon, was a rebel with a saintly dignity about her. At the Catholic church whose services Catherine would sometimes interrupt, a priest paid for her subscription to Amnesty International. George, the local greengrocer who would make her free sandwiches, remembered her as a cultured woman who always read the Times.
Some things began falling into place. Goodness was, even in a schizophrenic's mind, remembered. On the walls of Catherine's flat, among the frightful drawings, were two simple, scrawled sentences: "Thank you George" and, vertically, "Thank you God."
One more. On a wall near the sleeping bag that still carried the imprint of Catherine's body: "Lord, this cell is cold."
Mary Loudon cried when she saw that.
ANY REGRETS? I ASK, KNOWING FULL WELL THAT this is the moment when the voice will break or tears well up in the clear green eyes of the woman across the table from me. But Loudon looks straight back and says, without any catch in her throat, that if she'd had her time again she wouldn't have written Catherine such anodyne letters. "Even if I didn't get any back, I shouldn't have done that. I mistook her lack of response for a lack of ability to engage. When I looked back at her letters after her death, some of them were actually pretty jolly. I'd missed that because I read them through a fog of fear."
Loudon is, in fact, commendably composed and lucid in all her answers about Catherine. I should perhaps have guessed that from the book. Despite its subject, and for all its personal edge, it is balanced, thoughtful, and never strains for emotive effect.
"I can't stand that kind of writing," she says. "Sloppy confessional writing has nearly always got in it an element of prurience on the part of the interviewer and an element of exhibitionism in the subject."
She's quite vehement on the subject, and I hadn't expected that either. Such an empathetic writer would, I had imagined, be touchy-feely in person too. Instead, beneath the charm, one senses a certain steeliness. Her four books, after all, have had three publishers; she herself has pulled the plug on projected documentary follow-ups to them. For her book reviews for the Sunday Times, she had an agreement that not a word could be changed, and she seems suspicious of interviews in which (unlike her own) the interviewee does not have final approval.
But that kind of discipline brings its own rewards - and not just in the 100,000 advance she received from Canongate for Relative Stranger (she points out that it has nearly all been earned back through foreign sales). Ultimately, it's one of the reasons her book is so good in the first place: that instead of an ego-ridden confessional splurge, it's a brilliantly clear portrait of the havoc mental illness can wreak in a family's life, showing just how much of a personality it can occlude, but also the value of what's left.
"Within the confines of her illness, Catherine led the life she chose and made decisions on the things she wanted to do," she says. "Knowing that she had people around her who cared about her, and that she kept herself busy, were big things for me.
"I'm not a lazy person and am always occupied with something - whether it's my children or going out for my daily jog or swim or whatever. And in my twenties, I found the idea of Catherine not going out and meeting people very distressing. All through my twenties, and part of my thirties, that was my image of her, that she'd be rocking to and fro and looking at the wall. Actually, she produced thousands of paintings, she had people she called on every day, and she had created a safe routine for herself.
"I don't know why she wrote 'Thank You God' on the walls: and there's much more I don't know than I do. But I think she probably had moments of lucidity when she was closer to seeing what we see in the world around us, and from the people I talked to, she was happier than I had imagined. Much, much happier."
Relative Stranger by Mary Loudon is published by Canongate, priced 16.99.