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
The story opens with Cecilia's suicide and continues until all four of her surviving sisters are committed to their macabre mission of ending their own lives. What would make them do such a thing? Perhaps their stifling home life, where their mother doesn't even let them leave the house for school.
Act II
Now that the girls are under house arrest they escape the only way they know how: into the next life. Each sister sets up a personalized suicide method, and the other girls help each other out, so that they fall like a line of dominoes.
Act III
Mary, the last girl, survives her attempt and comes home from the hospital for one month. It's enough time for the parents to clean out the house and sell it, and for her to finally tie up the loose end of her life: she overdoses on sleeping pills and the Lisbon parents move away. Oh, the humanity…