What is the main conflict in The Virgin Suicides by Jeffrey Eugenides?

The book opens with the scene of an ambulance coming in to pick up the body of Mary, the last of the five Lisbon sisters to take her own life. The scene is observed passively by a few young teenagers gathered inside a house across the street. The novel then proceeds as a flashback, recounting with ample detail the 13 months leading to this final, dramatic outcome (suicides).