No, they are not.
My sister, Eve Kosofsky Sedgwick, the professor of English literature, writer, critic, and poet who sadly passed away a few days ago (April 12, 2009), acquired her last name through marriage to Hal Sedgwick, a researcher and professor in the field of the psychology of visual perception. Thus, any relationship to other Sedgwicks would only be through marriage. As far as I know, Kyra Sedgwick is not related to my brother-in-law, Hal.
David Kosofsky, New Orleans