The sisters collide and Mai is forced to realize that Maki has something she doesn't. She resents Maki for leaving the Zenin Family and feels the pressure of living up to be better than her. Despite Mai's best efforts, Maki shows her little sister the difference in their strength.
Just prior to the start of the Goodwill Event, Nobara asked Yuji what he thought about Maki. He was afraid of answering but Nobara means as a jujutsu sorcerer. Despite not knowing much about jujutsu, Yuji could tell she's a tough fighter just by her poise. Nobara revealed that she was only grade 4 due to her family but they could help get her noticed during the event if they do well in the Team Event.
Maki overheard them and recalls this at the start of her fight with Mai. Maki is skilled enough to slash apart Mai's bullets and cuts off her footing with just a sword. Mai attempts to fend her off by shooting but Maki evades and kicks Mai aside. This reminds Mai that her sister has something that she was born with, something that Mai does not.
In their childhood, Maki would help Mai ignore curses that appeared around their home. Mai was jealous of how carefree Maki could be, and this eventually lead to Maki leaving the family altogether. Their old man, Naobito Zenin threatened to make Mai's life harder but Maki demanded she stays out of this matter. Mai overheard this and called her sister a liar for leaving.
After deflecting Mai's sixth bullet, Maki believes her opponents revolver is empty. However, Mai despises her sister so she never shared the details of her cursed technique: Construction. This allows Maki to construct one bullet a day using mass amounts of cursed energy that puts a strain on her body. This allows Mai to take her sister off guard, but Maki's reflexes are fast enough to catch the bullet.
Mai believes that Maki has something she doesn't. Perhaps a reverse form of Mechamaru's Heavenly Restriction. Maki was supposed to be born with a cursed technique but she traded them for superhuman physical abilities. This was shunned on by the Zenin Family, but it's an ability that Mai does not have.
Defeated, Mai recalls her sister promising that she would never leave. She asks why Maki wouldn't live out life as a peasant with her instead of leaving her behind. Mai never wanted to be a sorcerer and felt like she had to because Maki went to chase her dreams. Desperate for answers, Mai asks why Mai didn't fall down the hole with her.
Maki responds that she would never have been able to live with herself and leaves her sister behind. Before being officially eliminated, Mai cries while calling her sister a liar and claims she hates her.