Is this the worst novel of Austen? I will say no, we have 'Northanger Abbey' to consider. But do remember, the worst of Jane Austen doesn't mean its a one starred book.
It might be the influence of 'Pride and Prejudice' that put out the shine of this one. The parallels are too obvious -- three sisters, one tricked by a villain and finally the happy marriage after tribulations. But the male counterparts here are rather weak and plot also lacks the perfectness of the former novel. Mood is grave and please don't attempt to read this immediately after 'Pride and Prejudice', you are going to hate it.
The three sisters are Elinor, Marianne and Margaret. The eldest one had a liking to Edward Ferrars, the eldest brother of Mrs. John Dashwood but as the family of his is not in favour, this comes to nothing (at least for now).
Then comes the Miss Steeles and the youngest one Lucy discloses to Elinor that she is engaged to Edward Ferrars for four years; the news is kept in secret for fear of his mother. Elinor takes the news valiantly and during winter they visit London with Mrs. Jennings. Mr. Willoughby is found to be engaged and Colonel Brandon reveals another treachery of his to his niece who has been molested and left pregnant. Soon the news of Edwards engagement comes to open and Mrs. Ferrars disinherits him as expected.
--- film (1995) is more interesting, especially the part of Marianne done by Kate Winslet. It won an Academy award and is nominated for six others including Best Picture.