Edna Strickland was the sister of Gerald Strickland and an etiquette columnist for the Hill Valley Herald back in the 1930s. Edna is the middle child of three, with an unnamed older brother. She shares the Strickland family trait of uncompromising morality and discipline. Unlike her younger brother Gerald, Edna was actually capable of smiling and even being civil at times. But for the most part the Strickland doctrine of strict discipline was still as much a part of her character as it was for her little brother.
NOTE: In Back to the Future: The Game, Edna ends up romantically involved with both Doc Brown and the Tannens, in different timelines.
Source: Futurepedia, "Edna Strickland Tannen," available under the CC-BY-SA License.