British author Rosamund Lupton's debut novel, Sister, probes the multifaceted relationship between two tightly bonded sisters, Beatrice and Tess, and the reasons one took so long to understand their mother.

How much did your own family relationships affect your writing of Sister?

My bond with my own sister certainly has been exceptional, and I consider it a gift, the kind you receive whether you deserve it or not. So I really, really wanted to explore this relationship, and when my youngest child went off to school about five years ago, I finally had time to start the novel.

You also present a complicated mother-daughter relationship through Beatrice's long letter to her murdered sister, Tess. How do the mother-daughter connection and the bond between the sisters interact?

These relationships climax when Beatrice, a career woman, and her mother visit Tess's grave. Beatrice realizes that while she had always sent flowers to their mother on the birthday of the little brother, Leo, they had lost—"thoughtfulness at a distance"—Tess, whose hippie London lifestyle Beatrice had deplored, had always come to see her mother on those sad birthdays. Seeing her mother become the "mum of babyhood" again through Tess's eyes opens a whole new consciousness about her own life for Beatrice herself. As her bond with her dead sister tightens, it also draws Beatrice closer to their mother, causing her to rethink her wedding engagement plans and even her career ambitions, because she now realizes she had entered into them for all the wrong reasons.

Clearly you are passionate about social injustices. Which of those that you address in this novel do you feel most strongly about?

Tess's apparently bohemian life, like her out-of-wedlock pregnancy, shouldn't have been judged harshly. Her fate teaches us not to make superficial judgments; outward appearances can often blind people to the true character of others.

Did you intend a play on the name of "Mr. Wright," whom Beatrice also addresses?

I invented him because Beatrice needed a vitally important listener, one that was almost too good to be true, to keep her narrative on the straight and narrow.

Sister closes on a remarkable surprise ending. Without revealing it, would you describe how you developed it?

I always knew it, because I pictured what Beatrice—or anyone—would do if she was so frightened.

What relationships will your next novel explore?

The next is a more complicated novel dealing with family relationships again, a mother who rescues her daughter from a burning building. It's almost done, and then I may take a bit of a breather.