First introduced as Sylvester Pemberton's adopted sister in in the "Star-Spangled Kid" (and Stripesy) feature in Star Spangled Comics #81 (June 1948):
She would soon figure out Sylvester's dual-indentity as the Star-Spangled Kid and develop her own costumed crime-fighting personae as the "Gimmick Girl".
from Star Spangled Comics #82 (July 1948)
And when Infinity Inc. was developed in the 1980s, it was eventually revealed that Merry was the mother of Brainwave Jr.