From what I can tell, I think it's quite good. In order for readers to give you more valuable feedback, you would need to give us more information, such as a synopsis of the story, or an explanation of context, or anything that would give us an idea of the greater picture. In my opinion, there are two ways to analyze writing: microscopically and macroscopically. Here, there is only enough to critique at a microscopic level.
On a microscopic level, the excerpt is well written. There are very few grammar or spelling errors and your word choice is vivid and precise. I particularly like this phrase: "...the demon’s mouth and tongue cold against the skin." You've chosen an image, which, without explicitly saying it, conveys the inhumanity of the demon and Nim's dread. Human mouths are warm; medical instruments and torture devices are cold. On a subconscious level, you've communicated that something bad is going to happen.
There are some places where the principle of "show not tell" could be better applied. For example, statements like "She didn't feel anything now. Emotionally at least" and "Curiosity claimed her" should be communicated using description and imagery if possible. Directly telling the readers what the character is feeling tends to pull readers out of the story, since that is not how we experience emotion. We don't go about our lives thinking "Wow, I am really curious right now." As an author, it is your job to convey human experience using the written word. Think: what does it feel like to be emotionally absent? What does it feel like to be curious? That is how you show not tell.
Another thing you want to do is make sure your description is completely original. Some of your description feels familiar, in that I've read similar situations being described in similar ways in other books. I'm going to paraphrase Robert McKee: Don't trust your intuition, because what you think is your intuition is the regurgitation of everything you've ever read. When you write, imagine the scene from the perspective of your character and describe what you see, hear, feel, etc. as honestly and sincerely as you can.
Despite all of the above, keep in mind that actual writing ability has very little to do with storytelling ability, and it is usually the latter that makes or breaks a story. So, I'm going to make an attempt to give you some macroscopic feedback. This excerpt, especially the first part, would make a great blurb. You've done a splendid job of building up tension and intrigue. However, all the questions you've raised (ex: Why is Nim going mad? Who are the Mad Sisters? How did Nim fall into their clutches? How was Nim saved? Who is the odd boy?) will have to be answered at some point. I also want to caution you about characters and caricatures. At this stage, the characters aren't very compelling because we don't know what their motivations are and we don't know what they want. This is alright, because you've only written two pages and you have lots of time to do that. In a lot of fantasy YA there is the stereotypical evil monsters and oddball sidekick. I want to caution you against merely regurgitating these tropes. Understand your characters, fully flesh them out, and portray them honestly, and they will be captivating.
All in all, great start, and I hope you continue.