Author Christa Parravani explores the bonds of sisterhood in her new book "Her: A Memoir".

Author Christa Parravani touches on the subject of the death of her twin sister Cara in her new book. Cara and Christa were identical twins. When Cara died of an overdose, Christa went into depression. The author's latest book, "Her: A Memoir", talks about what happened before and after the death of her sister, who she loved more than life itself, but was impossible to save.

Both sisters were artists in their own ways. While Christa took up writing, Cara was into photography. They had a tough childhood and grew up with a troubled mother. Things took an even worse turn when Cara was raped at the age of 24.

Christa talks to NPR about her life experiences:

On the unusual closeness of their relationship, including when Cara invited herself to Christa's honeymoon

"I wasn't angry. Even though maybe, you know, my former husband had wished I'd been a little bit more angry about it. ... She needed [him] to know that she felt as if marrying a twin meant marrying both of us."

On how the attack changed Cara

"One afternoon, she went out and walked her dog in a park near her home, and she met a man in the woods and he raped her. And it was a many-hours attack. She emerged a changed woman. I think that she was surprised that she survived that attack, and in a way she felt that she hadn't survived, that she'd left part of herself on the forest floor that day."

On Cara's resulting heroin addiction and the pain of watching her spiral downward

"I felt like she was eating me alive. I felt like there was no room for me to grow and to become the adult that I wanted to be. And I had always felt that to a certain extent in our twinship, because to be linked as twins, leaving the bond is something that feels like an act of infidelity. And I felt incredibly guilty."

On Christa's own decline following her sister's death

"I gave up in a lot of ways. ... There's so much narcissism in addiction. And in twinship, you're seeing yourself in that person and they're seeing themselves in you the whole time. It's this vicious circle of a built-in narcissism in twinship anyway. So when the addiction came in, there was absolutely no room left for me. And sometimes I thought, maybe it will be easier for me when she dies. But, of course, I learned very quickly after she died that that was not true. And I wasn't prepared for looking in the mirror, for example, and seeing my sister staring back at me."