She was enjoying his fame - talking readily of how she had sat him on the sofa and fed him buckets of icecream when he came to stay - but was wary of discussing his delicate psyche.
Wherever she goes, the people she meets are always trying to get an inside track on Daniel. Even Ireland, where she has a cottage in County Mayo, provides no relief. "The Irish are worse; they come to the door," she says. She sends the nosey packing.
Wary of being classed as such, I ask her about her work as a documentary producer. Seven years ago, she was forced to leave the BBC because of a rule against husbands and wives working together.
Sending out countless proposals and coping with the inevitable rejections has been tough. Although Tamasin's various series on adultery, gifted children and letters from Second World War soldiers have been well received, she lives in a constant state of insecurity.
Does she not envy her younger brother's success? "I would love to have people begging me to fill slots, just as he has piles of scripts waiting for him. And it would be useful to have more money," she admits.
"But I don't envy him his fame - not when I see what has happened to him. He has no privacy. Everyone feels they own a bit of him, that they have a right to know."
In the face of gossip, she says, Daniel maintains a dignified silence. "It's the only way," she believes. "I get crosser than he does and call up newspaper editors. I can't bear the way he is vilified."
The tale of how he dumped the French actress Isabelle Adjani - then seven months pregnant - by fax is "pure fabrication," she says.
Nor does she have a particularly warm recollection of Adjani. "When she came to stay at my home in Somerset, she never took her sunglasses off. She probably wore them in bed." Tamasin's nephew, Gabriel, is now two years old, and she has never set eyes on him.
Last November, when her brother finally married, another story illustrating his "callous" behaviour did the rounds. Deya Pichardo, the personal trainer who lived with him in New York, claimed that she had only learnt of the marriage from Adjani. This is bosh, says Tamasin.
"They had broken up months before; he was kind enough to let Deya stay on in the flat."
Sadly, the persistent image of Dan as the heartless Lothario has caused a rift within the family.