Check out this nice long snippet of City of Heavenly Fire. There's a great discription of Helen and Jules!
A moment later, Helen had returned; she was walking slowly now, and carefully, her hand on the back of a thin boy with a mop of wavy brown hair. He couldn’t have been older than twelve, and Clary recognized him immediately. Helen, her hand firmly clamped around the wrist of a younger boy whose hands were covered with blue wax. He must have been playing with the tapers in the huge candelabras that decorated the sides of the nave. He looked about twelve, with an impish grin and the same wavy, bitter-chocolate hair as his sister.Jules, Helen had called him. Her little brother.