Her grandmother, Abuela, is also a likely candidate. Abuela is the matriarch of the family and values honesty highly. If Mirabel lies, Abuela would likely take it upon herself to punish her to uphold the family's values.
It could be her mother. Mothers are often the ones who enforce discipline in the family. Her mother might feel disappointed in Mirabel for lying and give her a punishment to teach her a lesson.
Even her older sister Isabela could be the one to punish her. Isabela is seen as a role model in the family, and if she catches Mirabel lying, she might give her a punishment to make her understand that lying is not acceptable behavior in the family.