I had never thought about the Phyllis character that way before, but now that you mention it, Edward, it is a nice contrast from Ruth, and sort of shows that while Phyllis over-reacts to the slightest things, Ruth is very reserved, even though she has deep-seated anguish far more oppressive than anything Phyllis has had to endure. So in a sense, one sister has petty pain, yet responds to it mellow-dramatically, while the other has torturous pain, yet expresses it meekly. Well, until she reaches a breaking point and commits murder.
One thing I wanted to mention again is the music in the episode. It is very distinct, much like the themes to "Murder By the Book", "Ransom for a Dead Man", and "Any Old Port in a Storm"; these all have themes that are only heard in a single episode (although I believe "Ransom's" theme can be heard quietly as background music in a few other episodes). Also, much like the Patrick Williams scores for later shows, I feel "Old Fashioned Murder's" music really helps set the mood.