This story is so much fun to interpret, because it is so thoroughly human, tragic, and relatable, but it is also complex and rich. Here's one of the most rewarding interpretations of the plot: perhaps the novel is a picture of grief, and each character represents part of the pain that the father left the family with when he killed himself. In other words, although the characters are all complete and interesting, they all also represent something about Gilbert's own psychology. So, using Gilbert as a focal point, what would other siblings mean about his psychology?
Well, Arnie represents the psychological id, since Gilbert is always running around trying to stop Arnie from getting into trouble, and when Gilbert has his panic attack and he hurts Arnie, he also hurts himself very deeply. His honorable oath to protect Arnie is a picture of love, and if Arnie represents something about Gilbert, than Arnie's story represents how Gilbert struggles to love himself knowing what he knows about his father's depression and death. Arnie's constant attention-seeking is not so different than Gilbert's desire for love and adventure.
The girls represent different aspects of grief. Ellen represents the damage of the suicide in her rebellion and anger. Everyone knows that Ellen's cynicism and hyper-sexuality are signs of depression, but they often complain that Ellen makes things worse than they need to be because she is wrathful. That is certainly an understandable outcome from a parent's suicide. Amy represents the feeling that now, they should step up and be a family again to the best of their abilities, so it's no surprise that those two motivations clash.
Janice gets away, and so she represents Gilbert's longing to escape from his hellish daily life, even though he loves his family dearly. And Larry represents the trauma and lasting emotional pain of the suicide on all parties. His PTSD is understandable, but Gilbert also has PTSD that goes without remedy, and Larry represents that.
Ultimately, the death of the mother is tragedy and freedom at once. By coming to terms with the ultimate sadness (the death of one's parents), they free themselves up from the horrible complications of their dysfunctional lives, and they are free to reform their family in a healthier way. In this model of interpretation, this would signify the end of the grieving period, where the most obvious reminder of the husband's death (the obese mother) is removed.
