For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
Chapters 1-14. Elinor and Marianne are both involved in the relationships that will shape the rest of the novel.
Act II
Chapters 15-43. Happy endings for either Marianne or Elinor seem impossible, due to Willoughby's marriage, Marianne's dramatic illness, and Edward's coming marriage to Lucy.
Act III
Chapters 43-50. Everything works out for the best, and everyone finds their own kind of happiness (except for Willoughby).