

Lydia had inherited both her parent's dreams: Marilyn's dream of being a doctor and James' dream of fitting in and being popular. She thought she could do it by remembering Nath's instructions and encouragement from the first time she was in the lake, but swimming by yourself is harder than being saved.

She rowed out to the middle of the lake and jumped out of the rowboat and because she didn't know how to swim, she drowned, but accidentally. She decided to tell her mother that she didn't want to be a doctor and an over-achieving student, and to give herself the courage to do this, she decided to conquer her fear of the lake. (view spoiler) [We realize that Lydia did not commit suicide.

I don't think she absolves them of their sins, in fact it holds them directly accountable for them and drives their change. It is that last bit of irony that we really know what happens that pulls the story together. Each of the points in time above is when the character realizes that they did not know her, and from that point accepted their culpability and sparked their change. Overall it is sad, but everyone was better off thinking that she killed herself rather than it being an accident or killed. She was able to tell her mom that she didn't want her dreams (the cookbook), to tell her dad that she will be her own person and does not need to be popular (the cops telling him her friends never talked to her), and to tell her brother that she is OK and he doesn't need to save her anymore and he can be the person that he wants to be and she will not overshadow his dreams and achievements (through Jack and Hannah and the fight). Lydia in a way achieved exactly what she wanted, but of course she didn't what to die. (view spoiler) [Personally I think there was a lot of complexity in the ending.
