"Davidson herself has been rather luckier in love - indeed, she shrieks with excitement when I ask about her boyfriend of three years, Ben Jackson, who's six years older than her.
After rummaging about in her handbag, she fishes out a couple of cosily unglamorous photographs of them together; sitting outside in winter jackets, unselfconsciously cuddling. The way she talks about him seems to demand exclamation marks.
"He's so nice! And handsome! He sent me a bunch of orchids and lilies yesterday because he loves me! It was pretty much love at first sight when I met him on a blind date," she says.
"He was a model back then and he'd seen me in a bar and got hold of my number. He called me a few days later from Paris, which I thought was terribly romantic, and so we got together when he came back."
Yet Davidson's relationship has placed her much closer to the stormy centre of her sister's life than would be comfortable for most siblings. Not only do the couple live at a flat in one of Law and Frost's homes, but Jackson is Law's personal assistant.
As a result, he travels extensively with the actor and they have become close friends, which can't make the marriage break-up easy on Davidson. Even more spookily, Jackson looks so like his boss that there have been claims that he acted as a decoy to lure the press off Law's scent when the star was in New York, dining with his Cold Mountain co-star, Nicole Kidman.
"That was completely made up," Davidson says, crossly. "Ben just happened to be working at the house in London when the photographers saw him."
She also denies that Jackson's loyalty to Law has caused friction with her sister, and stresses that the division of property between Law and Frost won't unduly affect her own domestic circumstances.
"We live in the flat because it comes with Ben's job. If they decide to sell the house and we have to move out, it's no big deal - we'll just find another place together," she says. "Sadie and Jude's relationship doesn't impinge on our private life. I'm proud of Ben and the work he does."