The Girl Next Door - Augusta Huiell Seaman

Marcia Brett tells her friend Janet that the old, broken-down house next door, which they always thought was empty and abandoned, is apparently not so. An old, veiled woman lives there, and so does a pretty, blond girl of about their age. They discover that this girl, Cicely Marlowe, came to live with old Mrs. Benedict after her mother's death, but she doesn't know why, or she is. All she does know is that she must remains mostly unseen, not look out the windows, or talk to anyone.