They might be really surprised. Some characters could be flattered by the positive portrayals in the fanfiction. For example, Mirabel might be happy to see different interpretations of her story and how she's seen as a hero in some fics.
Jaina Antonio was a character in the movie Fast Track, played by Claudia Giellini. She was the leader of the Camorra organization and the leader of the mafia. In the movie, her brother, Santino Antonio, planned to kill her to take her place at the high table. Jaina Antonio also appeared in the follow-up movie, The Fast Hunt 2, played by Claudia Gerini. Information about Jaina Antonio's character in other works or games was not available in this search.
Encanto is about a magical family living in a hidden place in Colombia. Each family member has a unique gift, but the main character, Mirabel, doesn't have one. The story follows her as she tries to save the magic of their home.
Encanto is an original story. Disney's team of writers and animators came up with the concept and brought it to life on the big screen. It's not a adaptation or retelling of another story.
Indeed, Encanto is based on a story. The creators combined different imaginative concepts to bring this story to life, featuring themes of love, unity, and self-discovery within a magical setting.
We don't really know for sure without more context from the fanfiction itself. But they could be two friends, or perhaps they are characters from a particular series or universe that the fanfiction is based on.