In an interview with the Lady magazine, she reveals that for 12 years she had to make appoinments to see her mother, now Baroness Thatcher, through her secretaries - and that she still speaks to them far more than with her mother.
In the interview, the 42-year-old journalist admits there was intense competition between herself and Mark Thatcher and says she behaved well because of his "appalling" behaviour.
"In some ways he has behaved appallingly. I tried to behave in a vaguely respectable fashion because Mark got such awful publicity," she says, adding: "I didn't want everyone saying: 'Mrs Thatcher must be awful; both her children are off the rails'."
The interview suggests that what did not end up in print may have been even more frank: "Despite being twins, Carol and her brother have never had any special affinities; in fact, 'competition was intensified by the narrow age gap'. They are, she says, very different."
While Mr Thatcher's business affairs and society marriage are closely followed in the media, she lives modestly with her boyfriend, a ski-instructor, in Klosters, Switzerland.
In what may be interpreted as a veiled swipe at her brother's perceived lack of charm, she says: "I never had the opportunity to become high-handed, because I had to go on earning a living as a journalist. Unlike my brother, I have never made swags of cash."
She also tells how, for 12 years, she communicated with her mother through secretaries. "I always had to make appointments to speak to her. I didn't find it difficult because it was absolutely the norm. Some of the secretaries were my age, so they were the sisters I never had," she says.
This is not the first time Ms Thatcher has suggested a less than symbiotic relationship between her and her twin. She has previously referred to the fact that they are "not close", and in an interview before the publication of her biography of her father, Denis, she said she felt "second best" to Mark.
"Unloved is not the right word ... but I never felt I made the grade. Mark was a blond, very attractive little boy, and sporty, so Dad was always teaching him to play cricket on the lawn... I always felt I came second out of two," she said.
But it seems her brother's activities were not always unwelcome. In her father's biography, she refers to the time when Mark went missing during a desert car rally. "The episode did have one benefit," she wrote. "We could relax a little, for Mark had hung an occupied sign on the family's 'embarrassing relative' slot."