The death of her sister Helen has a very clear impact on Lily in many ways and had long reaching effects for years after the event. First of all, Lily never had much confidence in men especially after her disasterous marriage to the "crumb-bum" bigamist in Chicago. She stayed away from men and kept those that she thought might be interested at arms length so as to not have any chance to suffer that emotional pain again. Her attitude about men was only reinforced when Helen came to stay with her in Arizona after being dumped, preganant and alone, by some Hollywood slick kind of man who wanted nothing more to do with her after finding out about the baby. Lily took her sister in and tried to help her figure out how to go on after this emotional blow, but when the townspeople found out about the unwed mother, Lily's reputation suffered for it. Lily didn't care what people thought and she perservered as always, but Helen wasn't that strong and didn't have the emotional strength to endure the life she was going to have to face and the disappointment her parents might fee,l so shortly after Lily thought they had a plan in place to handle it all, Helen took her opportunity and hanged herself while Lily was out running an errand. Lily was so shocked and angered by what had happened and why, but she did the best she could do right by Helen, even burying her in Lily's favorite red silk shirt that Helen had always admired.
After Helen's death, Lily was especially bothered by the fact that both her sister and the unborn baby had died, so Lily decided it was time to try marriage again and to have a family of her own. She approached Jim Smith, whom she was already good friends with, and proposed marriage. She went into THIS marriage with a mindset of practicality, control and equality -- she would never let a man do to her what her first husband did or what the man from Hollywood did. She ended up having a very strong, loving marriage and two great kids. Thoughts of her sister and what happened to her were never far from her mind though, especially as Rosemary grew up to be a beauty and to be a woman with a mind of her own. Lily always worried that Rosemary would end up in ruin, just as Helen had. All that happened to and because of Helen stayed with Lily her whole life.
I would have to say that it would have to be the suicide of her younder sister. After that tradegy it is the first time that you really see Lily sinking for longer than a moment. It takers her a while to come back from this one and when she does she has changed- wanting to be a mother to make up for the loss of her sister.