deaths haven't. There is no one else for Junior to lose. He has lost the people that have always been on his side, the ones who have shown unconditional support. Mary's death is an emotional rock bottom.

Her death also leads to the falling action of the story which could be defined as the fight with Rowdy and the family's time at the graves of the dead. The time at the grave brings Junior's family together and the fight with Rowdy leads to the resolution of the story where all the loose ends are tied up. At the end, Junior and Rowdy reconcile their differences and become friends again. Rowdy accepts Junior for who he is and wants to be, and they play ball to close out the story.

Mary's death allows Junior to accept his life, his fate and bring the remaining people in his life together.

Sherman Alexie’s novel can be read as having several climactic moments rather than one major climax. One of the most significant climaxes comes when Junior’s grandmother dies. Up to that point, he has struggled with identity issues pertaining to his decision to attend a high school off the reservation. Most of the other teenagers disapproved of his decision, considering him a race traitor. During the basketball game between the two high schools, Junior regained some respect, even though he was on the opposing team, because he tried his best. Nevertheless, he is still largely ostracized by his peers.

Junior’s grandmother was a well-respected elder on the reservation as well as a powerful figure in his life. When she is killed in an accident caused by a drunk driver, the entire reservation community comes together in mourning and makes a massive showing at her funeral. The magnitude of his loss is evident to everyone, and the other community members realize that he needs the support of his people. His loss and grief provide the necessary reminder to the other reservation residents that he is still part of the community. In addition, a wealthy white man becomes an object of ridicule when he falsely claims to bring the grandmother’s valuable dance outfit. This tragedy and the unexpected comic relief both help re-embed him within the community, and from that point forward he is not ostracized.

The climax of The Absolutely True Diary of a Part-Time Indian occurs when Junior's sister Mary dies.  Junior has already lost his grandmother and his uncle Eugene, so the death of his sister makes him feel like all is lost.  During this chapter, Junior also loses Rowdy as a best friend.  In order to deal with the loss of Mary, people around Junior get drunk.  He hates it, so he leaves the reservation and goes to school.  He finds that the people there care for him and are concerned about his well-being.  They ask him how he is doing and Penelope cries for his loss.  At this moment, Junior feels like "every planet in [his] solar system has exploded." 

After this point, Junior reconciles with living on the reservation and seeing the beauty there.  He also regains a sense of pride in who he is.  In the end, he and Rowdy become friends again.  This is the falling action and resolution of the novel.

I agree that there are many great scenes in the novel.  I think the most important scene is at the end of the novel when Rowdy and Junior have become friends again.  One of the major conflicts that Junior faces--in addition to his difference from those at Reardan--is the conflict and pressure that he feels from those on his reservation after he decides to leave to attend school in another place.  Junior is seen as a bit of a "sell-out" after he makes this decision, and he is immediately alienated from his own community.  This is incredibly difficult for Junior to deal with, so when he and Rowdy become friends again, a significant resolution is reached.  Junior sees the beauty that the reservation has to offer, and Rowdy understands why Junior decided to aim for what he saw as a better opportunity.

Any number of scenes could provide a variety of answers to your question, but I'll offer one.

Junior's first day at Reardon is certainly an important event and transformation in the novel. In this scene, all of the apprehensions and anxieties of crossing the divide from one culture to another come to fruition. Junior is initially ignored by the white students but the taunting and threatening remarks soon follow. Of course, these tensions continue to boil within Junior an eventually he is driven to violence is retaliation.

So much of this novel is about living caught between two radically different worlds. This scene when those tensions become a physical reality is certainly one of the novel's most important.

In a story, the inciting incident is the event that draws the protagonist into the main conflict, disturbing their everyday world.

In The Absolutely True Diary of a Part-Time Indian, the inciting event occurs after Junior gets in trouble at school for throwing the geometry textbook at Mr. P. Rather than getting angry, Mr. P. apologizes to Junior, telling him that the reservation school is not serving him or any of the students well. Mr. P feels the poverty and hopelessness on the reservation tend to crush the spirits of young people, no matter how intelligent and ambitious they are. He uses Junior's smart sister Mary as an example of this phenomenon (though she is a talented writer, she stays in the family basement all day) and advises Junior that if he wants to ensure his future will not follow the same downward trajectory, he should try to leave the reservation as soon as possible.

The meeting with Mr. P. inspires Junior to take action. He decides to go to the local all-white school in order to obtain a better education, even though he will be an outsider there. This action pulls Junior into the main conflict of the novel, which sets Junior against both the prejudices of the all-white school and the prejudices of his reservation home.

In any coming of age work, there are many defining moments because the compendium of the work based on a life consists of "defining moments."  Those are the ones that entered into the work. This means that a case can be made for any moment featured in the novel as representative of one that is definitive.

I think that the rematch between Reardon and Wellpinit is probably my vote for the defining moment.  Junior and Reardon entered the game on a 12 game winning streak, with their sole loss at Wellipnit.  The loss was marred by a hard foul on Junior by Rowdy, as well as animosity from the crowd aimed at Junior.  At the same time, Junior himself had overcome the difficulty of transitioning to Reardon, telling his Social Studies teacher who is insensitive to his predicament his own conception of the world:

...the world is only broken into two tribes...people who are assholes and...people who are not.

With this realization in hand and buoyed by an increased confidence, Junior plays the game of his life against the Reservation school and against Rowdy.  He vows to never acquiesce to anyone and the game becomes the forum where this mantra is put to its first and ultimate test.  He succeeds and Reardon slaughters Wellpinit.  During the celebration, Junior sees the inescapable sadness that seems to follow the Native American players even away from the reservation.  He understands the differences between both worlds and in the midst of his greatest happiness, he retreats to the locker room and cries.  His tears are representative of how he has a foot in both worlds, and how the dual consciousness he experiences in both will forever help to form his identity and always be a part of him.