Ann Patchett's writing is all charm and grace and soothing. Here we are at a party. We're celebrating a new birth. Everyone is hanging out together, having a drink. Isn't it lovely?

Then you're pulled under by a sneaky riptide of wild during a banal event, like a smack of instant attraction sweeping in while changing the diaper on a baby.

Patchett, who has built a following with superb novels such as Bel Canto, will appear with writer Jacqueline Woodson, author of Another Brooklyn, at the Philadelphia Free Library at 7:30 p.m. Oct. 6. The auditorium is sold out. Patchett's latest novel, Commonwealth, tells of two families of siblings, joined as one by betrayal and divorce, dominoed by a kiss at that party. Her description of the sibling relationship is delicious, love with a nice hot splash of envy and jealousy. It follows the six step-siblings over 50 years as they squabble and grow up and find a place in their lives. Watch for that undertow.

From her home in Nashville, where she lives with her husband and dog, Patchett talked of layered stories, the roles siblings play, and why she doesn't recognize her own writing.

I love how you describe siblings. It's rarely a simple relationship.

Complex is the right word, or layered. I based these relationships on the relationships I have with my siblings. It's funny: When I meet people, I ask about their siblings. Well, I ask about their children, but I tend to ask about their relationships with one another. And then I ask you if you have any siblings.

Did you talk to your siblings about the book?

I didn't interview them. But I cleared it with them because it is sort of about us. I sent it to them when I was finished, and they were fantastic and loving and supportive and wonderful. They also said, "This made us sick, and it was really hard to read and caused trauma for everybody." But they also said, "We love you, and you go, girl."

You've written about your marriage. You've written about your friendships. Why write about your family now?

I was trying so hard not to write this book my whole life. I'm 52 now, and I'm at the point where I have that kind of access to myself, and I want to grow, and I felt like I'm not going to grow unless I get this written.

You write a lot about how each sibling tends to have a specific role in the family. What was yours?

I was always the favorite. We had different parents, and we had so many parents, and all the parents had different favorites. I was a real pleaser. I got along with everyone except for my sister, and my sister and I really fought. And now my sister is my closest friend. So we had an above-average amount of difficulty between us growing up and yet were very intertwined. We never, ever just walked away from each other and just kept fighting. Then in our 30s, the switch was thrown and we're very close.

When you look back at your books now, how have you changed as a writer?

I've never read any of them. About 10 to 15 years ago, I was giving a talk, and the person who introduced me read a long passage from The Patron Saint of Liars. It was like I'd never heard it. If you had given me a test of 10 passages and asked me which one I'd written, I would not have thought I'd have written that one.