WALLING AVENUE FATALITY Oneonta Fireman Died Saving Kids, Sister-in-Law Says Erika Heller posted this photo of her brother-in-law, John Heller, on Facebook as she described how the former Oneonta firefighter sacrificed his life to get her sons and his fiancee, Amber, to safety when the fire broke out at his Walling Avenue home. By LIBBY CUDMORE • Special to AllOTSEGO.com Heller became trapped on the upper floor of 5 Walling Ave. after helping his nephews and his fiance escape the fire. (Ian Austin/AllOTSEGO.com) ONEONTA – John Heller, the former Oneonta firefighter who died when his home at 5 Walling Ave.…