Why was Oliver blamed for his mother's death in the novel, Vera Wong’s Unsolicited Advice For Murderers?

Early on, it is revealed that Oliver’s mother died of a freak accident while making his favorite dessert, something neither his father, Alex, nor Marshall ever forgave him for. This caused Alex to blame, and as a result, ignore and often neglect Oliver, giving Marshall the freedom to belittle and blame Oliver for everything.