I'm pretty sure that Janie is really the daughter of Peter and Phyllis, because Columbo mentions that Phyllis and Peter Eloped, and that Phyllis was 3 months pregnant at the time. So it was a bit of a scandal that Phyllis and Peter were having an affair while he was engaged to Ruth.

I agree it seemed very out-of-character for Ruth to frame Janie, but I guess it goes to show just how evil and desperate she really was.