Perhaps Abuela believes that Mirabel's presence without a gift is somehow jinxing the family's magic. In the fanfiction, she might think that by imprisoning Mirabel, she can stop the negative effects she thinks Mirabel is causing. This could be related to some superstitious beliefs within the Encanto about the balance of magic and the role of each family member.
In the fanfiction, it could be that Abuela is misinformed. Maybe she's been told by someone that Mirabel is the cause of the cracks in the Encanto. Without fully investigating, she imprisons Mirabel out of fear for the safety and future of the family. She's so used to having a structured family with everyone having a role according to their gift that Mirabel's situation throws her into a panic and she takes extreme measures.
Mirabel could use her wits. Since she has a strong will and determination, she might find a weak point in the place of imprisonment. For example, if it's a locked room, she could search for a hidden key or figure out how to pick the lock.
She might be heartbroken at first. After all, being kicked out by her own abuela is a big blow. But then she could become determined to prove herself. So she might set out to find a way to show that she is an important part of the family even without a magical gift.
Perhaps she tries to understand Abuela's perspective. Mirabel is a kind - hearted girl, and she may think that there is a reason behind Abuela's actions. So she might try to see things from Abuela's point of view while still feeling the pain of the punishment.
The younger characters in Encanto like Antonio would be excited. He loves stories and new ideas, so Mirabel fanfiction would be like a new adventure for him. He might want to share it with his animal friends. And Dolores, with her ability to hear everything, might already know about the fanfiction. She could be either intrigued or a bit worried depending on how the fanfiction is being received by the rest of the village. If it's causing a stir, she might be concerned about the family's reputation, but if it's positive, she would likely be quietly supportive.
Mirabel might be initially shocked and heartbroken. She loves her family and the village, so being banished would be a huge blow. She could cry and try to plead with Abuela to change her mind.
One possible reason could be that Mirabel might have accidentally broken an important family heirloom. In the context of the Encanto family, their heirlooms are often tied to their magic and family identity. If she damaged something like that, Abuela, who is very protective of the family's traditions and magic, would likely punish her.
At first, Mirabel might be shocked and confused. She would probably wonder why Abuela would do such a thing to her. She could spend her initial time in prison trying to figure out what she did wrong or if there was some mistake.
One possible reason could be a misunderstanding. Maybe Abuela saw Mirabel doing something that she misinterpreted as a threat to the family or the magic. For example, if Mirabel was exploring a forbidden area in the house where the magic was acting strangely, Abuela might have thought she was endangering the family's future and thus imprisoned her out of fear.
In the fanfiction, Abuela may be under a lot of pressure to keep the family's magic and reputation intact. When something goes wrong that she blames on Mirabel, she kicks her out. Mirabel then has to face the outside world alone. She could encounter various challenges and adventures, which in turn make her grow. Eventually, she might find a way to reconcile with her family and even help save the family's magic in a new way.
It could also be a misunderstanding. Maybe a magical accident occurs and Abuela wrongly blames Mirabel for it. Since the family's magic is so important to Abuela and the community, she might overreact and banish Mirabel without fully understanding what really happened.