"We were completely enmeshed in each other," Joely, 44, tells the January issue of Britain's Tatler magazine. "I hadn't lived a day in my life without her. You worry about your children and your elders, but I never imagined such a thing as this. Tash was a given to me."
It was the first time the Nip/Tuck actress has spoken at length about the sudden loss of her sister earlier this year. Natasha died after a skiing accident in Canada in March.
"The shock shatters you on a cellular level, and it takes time for the pieces to come back together, albeit in a different formation," she said. "I cannot imagine that there will ever be a day when I don't think of Tasha."
The actress has thrown herself into acting work, most recently finishing her filming of the latest season of The Tudors alongside Jonathan Rhys-Meyers.
But for her, part of getting over her grief is learning to accept it.
"I don't look at happiness as being a great goal. A sense of peace, definitely, but it's like the seasons," she says. "I think we're meant to have a spring, summer, autumn and winter. I'm not looking for a perpetual summer because that would be a flat line."