Heilmann's older brother, Walter, was a gifted athlete who attended Sacred Heart Cathedral Preparatory (then "Sacred Heart College") in San Francisco. On June 3, 1908, a year after the city's devastation by earthquake, Walter died at age 16 while on a sailboat excursion with three schoolmates to visit the battleship "USS Connecticut" in drydock at the Hunters Point Naval Shipyard, near the present site of Candlestick Park. The boat capsized in a squall, and Walter drowned while trying to swim to shore; the other three boys were rescued.