"I've got a firecracker feeling," sings Jocelyn Arndt, lead vocalist in the brother and sister duo, Jocelyn & Chris, headliners Saturday night at Jam at the Ridge, and she certainly does. Jocelyn lit up the stage while Chris laid down some tight groves and slinky guitar riffs during a high-energy set on a clear summer evening. The Arndts have been writing songs together since