Mirabel is the reason Antonio got his gift
So we know that the problems with the family's magic began when Mirabel tried to receive her gift, but was unable to. When her door disappeared, the candle flickered.
Ten years later, Mirabel was clearly considered to be bad luck to Antonio's gift ceremony, being pushed aside and told to stay out of the way. There was a fear that the problems that plagued Mirabel during her ceremony would reappear at Antonio's ceremony. So no one, especially Abuela, wanted to take any chances. But when it came right down to it, there was no one Antonio wanted with him at his ceremony more than Mirabel.
And Mirabel agreed to take him to his door. She acted as the support he desperately needed. This kid was already chronically shy, and he also knew the pressure that was on him to receive a gift. So he needed the one person he knew would be there for him and never treat him any differently if his ceremony failed.
But for Mirabel herself, this was a walk of shame. She had to relive the trauma of being publicly humiliated at five years old, and she had to do it despite the disapproving looks of guests and even family members. But she faced it all anyway, for Antonio.
We know that the problems with the magic were not just a fluke or resolved in any way by the time of Antonio's ceremony, as the rest of the movie happened. The cracks in the house started showing up that same night, while the party was still ongoing. The candle still flickered. The magic was still weak. But in the moment Antonio touched his doorknob, the magic was strong.
The strength of the family's miracle is determined by the strength of their family bonds. By facing her own trauma in order to help Antonio, Mirabel strengthened the magic itself so that Antonio could receive his door and his gift. Had he gone alone, or had his parents or siblings escorted him, his ceremony almost certainly would have failed. The success of Antonio's gift ceremony depended entirely on Mirabel and her willingness to put all her fears and desires aside in order to give her little cousin the loving support he needed. Mirabel's courage and selflessness gave Antonio his gift.
