Subseries consists of correspondence between Cornell and various members of his immediate family, including his mother, Helen S. Cornell, his sisters, Betty Cornell Benton (often addressed as Nell) and Helen Jagger, his nieces, Sissy (Jagger) Batcheller and Jean (Jagger) Plitt, and various cousins. Some printed material, primarily clippings sent to Cornell by his mother while she was living in Westhampton during her last years, can also be found. Subseries also consists of correspondence between and amongst other members of the family, including many of those listed above, as well as Cornell's brother, Robert (subseries includes only correspondence from him), his father, Joseph I. Cornell, and his cousins, Grace Sayres and Janet (Voorhis) Traphagen. Correspondence received by Robert Cornell can be found amongst his papers in Series 11.

Cornell's family correspondence documents his close and on-going relationships with his mother, sisters, and nieces; the large role that his family played in both his personal and creative life; the many interests he shared with his siblings, including art, literature, music, and Christian Science; and his collaborative work on collages with his sister, Betty. The one important family relationship not explicitly documented in the correspondence is Cornell's relationship with his brother, Robert. Since they lived together up until the month before Robert died, they did not exchange letters in a regular fashion. Some cards and notes between the two are scattered amongst Cornell's personal papers and diaries, as well as amongst Robert's papers. To a certain extent, Cornell's diary entries also document his relationship with his brother.

Family correspondence files are arranged alphabetically according to the surname of the correspondent. Items within the files are arranged in rough chronological order.