Basic plot of this book was, a teenage girl finds out she was adopted. Here are some specifics:
Girl had a younger sister, who was their parents' natural born child. Younger sister's name was Martha.
How the teen girl learned of the adoption was this way: Martha had just returned from a visit to the parents' hometown; and was describing to teen girl a conversation she (Martha) had had with an elderly man who had known the parents before parents had moved...The man had made comments that led Martha to put pieces together, and in the scene I do remember, Martha is telling teen girl about this conversation. Teen girl at first thinks the man was saying Martha was adopted, but Martha replies with the following quote (this I do remember verbatim):
Me? Me? Anderson was talking about a child adopted before Mother and Dad moved away from Milford. I was born here. You were born in Milford. He was talking about you. You're the adopted one!
Teen girl has a hard time dealing with this and leaves parents' home for a while. She was staying with a friend whose name was Charlotte. She does eventually find her birth mother, but don't remember how that ended up.
Does any of this ring bells for anyone? I am pretty sure I read this anywhere between 5th to.8th grade, which timewise would be from 1973 to 1977.