In this fanfiction, Hermione might be shocked when she finds out about the cupboard. She could start an investigation to figure out what's been going on in there. Maybe it's a hidden place full of secrets related to Hogwarts' history.
There could also be some unknown characters in the fanfiction. Maybe a secret society within Hogwarts that has been using the cupboard for their own purposes. When Hermione finds out about it, she gets caught up in their affairs. These unknown characters could be students from different houses or even former Hogwarts students who have come back for some hidden agenda related to the cupboard.
He could be furious. In many fanfictions, Snape is a strict character. Discovering something about a cupboard might lead him to suspect some sort of rule - breaking or hidden secrets. He might start an investigation right away to find out what's really going on.
She might be initially defensive. Hermione is a proud and independent character. So, when Snape finds out about her arm, she could be worried that he'll use it against her in some way, like taking away house points or using it as an excuse to give her extra work.
The truth could also be related to his feelings. Perhaps he's in love with Hermione more deeply than she realizes, and there are some misunderstandings that get cleared up when she finds out. In certain fanfictions, Ron might have been sacrificing his own opportunities for Hermione's sake, like giving up a chance to study with a great wizard so she could go instead.
One great fanfiction with this theme is 'Hermione's Pureblood Revelation' on FanFiction.net. It delves into how Hermione discovers her pureblood heritage and how it changes her relationships at Hogwarts. She has to deal with the new - found knowledge and the expectations that come with being a pureblood in the wizarding world.
One consequence could be that Hermione has to face the Malfoy family. They might try to bring her into their way of life, which she has always opposed. It would be a struggle for her to keep her own values intact while dealing with their pure - blood traditions.
However, if the truth is something more complicated or even a bit negative, like Ron has been lying about his family's financial situation to seem more worthy of her, it could cause a rift. She might feel betrayed at first, but depending on how Ron explains and if he's truly sorry, they could work through it.
She could be shocked at first. Just imagine, she thought she knew her family all this time, and then bam, she's adopted. She might be angry too, especially if she feels she's been lied to. But also, she could be curious and eager to find out more about her real family.
In some fanfictions, Hermione might find out she is a Malfoy due to a spell gone wrong. For example, a spell that was meant to reveal family lineages got mixed up and suddenly she has a connection to the Malfoy family in the magical world created by the fanfiction writer. It could also be a result of a hidden prophecy that links her to the Malfoys in an unexpected way.