Jessamine Milner was born in Cheyenne, Wyoming in 1894 and didn't start acting until 1972. There's not much about her on Ye Olde Internet, but I did find this heartfelt praise for her role in Sybil:

This was a deeply harrowing movie to watch, and unbelievably so when it came out in 1976. A small child in the grip of her homicidally insane mother, who inflicted sadistic torture on her, while her ineffective husband looked the other way when the signs of abuse were obvious.

There's a small performance in this movie that haunted me more than almost anything else in the film; the part of the grandmother, played by Jessamine Milner, who was as much a victim and prisoner in the home of her psychotic daughter as Sybil was. The difference was she was aware of the extent of her daughter's insanity.

What must it be like to be a prisoner in your own adult child's home, knowing she is inflicting abuse on your grandchild and will do the same to you if you speak? That kind of helplessness must be sheer hell to live with. She could have told her son-in-law or the police at any time (if she was able to get out of the house), but would they have done anything? Or turned a blind eye, considering the time?

Jessamine Milner's performance was so honest and affecting, it stands out as one of the most painful parts of the film, and she is in only two minutes of it! She was born in 1894, and was almost 80 when she made the film. She apparently was in her mid-seventies when she went into film! She's a mystery, and other than her few TV appearances in the late 70s, nothing apparently is known about her. However, she deserves a mention somewhere because of her performance in this difficult to watch film.