She may cry when she realizes the depth of her feelings for the Beast. It could be a moment of self - discovery and confusion. She is falling for someone who is not the typical prince charming, and this realization might be overwhelming for her, causing her to shed tears.
Perhaps she cries because she sees the potential in the Beast but is frustrated by his stubbornness. She might be crying out of a sense of helplessness in trying to break through his tough exterior. Also, she could be crying due to the general strangeness of her situation. Being in a castle full of talking objects and living with a Beast is not an ordinary experience.
At first, it might scare him. The Beast is not used to dealing with such emotions. He has been alone for so long that seeing Belle cry could make him feel out of his depth. But as he gets used to it, he learns to be more empathetic and it helps him grow as a character.
Belle could get sick due to the drafty corridors of the castle. The Beast would be beside himself with worry. He might sit by her bedside all day, reading to her from his favorite books in his attempt to comfort her. As she recovers, she might find new aspects of his personality that she admires.
Perhaps Wilbur is missing someone. He might have a friend or a relative who is not around anymore. In the world of 'Meet the Robinsons', there could be some backstory where he had to say goodbye to someone he loved, and at a certain moment in the fanfiction, those feelings of loss resurface and make him cry.
Perhaps Belle gets sick from eating some spoiled food in the castle. The Beast is panicked because he fears losing her. He decides to go on a dangerous journey to find a special herb that is said to cure all ailments. While he's gone, Belle is being taken care of by the enchanted objects in the castle. They tell her stories to keep her spirits up. When the Beast returns with the herb, Belle is on the mend, and their bond becomes even stronger as she realizes the lengths he would go to for her well - being.
This idea could lead to exploring how the Beast's perception and relationship with Belle might change. Maybe it would make their connection more profound as it wouldn't be based on outward appearance.
There are several potential causes. Perhaps the Beast's temper gets the best of him for a moment and he accidentally causes her harm, although he would be extremely regretful afterwards. Or, there could be a new, malevolent character in the fanfiction, like a dark spirit that lurks in the forest around the castle and attacks Belle when she ventures out. It could also be that she was trying to help the Beast with something related to his curse and something went wrong, resulting in her getting hurt.
It could be Belle's sister herself. If she's envious of Belle, she might turn into a villain. She could try to keep Belle away from the Beast and the castle forever, perhaps by locking Belle up or convincing the villagers to keep Belle away. This would make her the main source of conflict in the story.
Well, one possible plot could be that Belle's sister is jealous of Belle's relationship with the Beast. So she tries to sabotage it by spreading false rumors in the village about the Beast being a real monster who can't be tamed. This causes a lot of unrest among the villagers and puts Belle in a difficult position.
One possible plot could be that Belle's daughter discovers a hidden part of the castle that holds a secret about her father's past as the Beast. Maybe she accidentally stumbles upon a locked room filled with old portraits and journals that reveal how he became cursed in the first place. This discovery could then lead her on a journey to break a remaining part of the curse that still lingers on the castle and its inhabitants.
It could lead to a very different story. Maybe Gaston would change for the better with Belle's influence, or they might still have a lot of conflicts.