''I was one of the Williams sisters when we were known for our gospel singing, and for our barbecue,'' says Ernestine Van DuVall, a queenly woman with a deeply melodic voice at age 78. In the cool of her senior citizen's condominium, sitting next to the kitchen table loaded with pans of deep-dish peach pie, sweet and crispy summer pickles, fresh yeast rolls, and slabs of barbecue wrapped in foil, Ernestine begins to tell the story of how she got into the barbecue business: ''We started out in the Depression when there wasn't anything but jackrabbit, so we barbecued that. Later, we barbecued pork after we butchered hogs in the fall, but we still did jackrabbit.'' Ernestine uses her own spice rub and slow-smokes the meat. ''One year we were expecting so many people for the Emancipation Celebration,'' she laughs, ''that me and Charlesetta bought up every chicken between here and Nebraska for barbecue!''