Request #150

The one I can’t seem to find is a book written in the 70s or early 80s, and it was about two sisters, one of whom was hidden from the other because she had a disability and was a slow learner. I think the main character might have been named Cathy, but I can’t find anything when I search YA with her name. I don’t know the title or the author.  I think the mother was a stepmother. I was discussing old book favorites with a friend and we both recall reading some book like this from our school library (in the 70s/80s), but can’t recall the rest. Suggestion: Cathy At the Crossroads by Nancy W. Faber