Inez Foxx - born 9th September, 1942 (Greensboro, N. Carolina)
She didn't just record Mockingbird - this was a minor UK hit (#40) in 1964 - a first for UK Sue. Her brother Charlie unfairly didn't get his full name on the label. Later in the year they supported the Stones on a UK tour - Allegedly, Mick stole some of his on-stage moves from Charlie.
http://www.youtube.com/watch?v=aNHjMICHKxM
Charlie passed on in 1998. It does not appear that Inez has been heard from since 1974.