Helen and Esther Johnson, b.1911 and 1914,Illinois

Hi! In the search of my mother`s grandmother, Agnethe Andersen Ringnes (later named Agnes Johnson, married to Emil Johnson) I have found their children (my mother`s aunts): Helen C. Johnson, b 1911 in Illinois and Esther Johnson, b 1914 in Illiois. In 1930 they lived with their parents (Agnes Johnson and Emil Johsnon) in Marinette, Wisconsin. They worked in the hospital. I wonder how I can search further to find out more about Helen and Esther? Per Jakob Haugaa