Seraphina

Lady Aldworth is looking for someone who will give her grandson an heir, is meek, content to let others do her thinking for her, and is ready to allow Lord Aldworth to continue in his present mode of life.

If acting like a brainless ninny would net Serafina a wealthy husband, and so get her family out of difficulty, she would do it - until Charles caught the real Serafina in action. The only recourse was to pretend to be her 'sister' Sally...