Chapters 1-14. Elinor and Marianne are both involved in the relationships that will shape the rest of the novel.
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.
Chapters 43-50. Everything works out for the best, and everyone finds their own kind of happiness (except for Willoughby).