Beth Wincander will never turn 13.
As others have mentioned, this is a short, well-crafted hook.
When a witch stole Beth’s cousin three years ago, Beth discovered her family was cursed: for the last 23 years, all Wincander children disappear in the eve of their twelfth birthday to not be seen again.
Is it necessary to state the exact number of years the curse has been on the family?
Here’s my rewrite:
Three years ago, a witch stole Beth’s cousin; that’s when Beth discovered that children in the Wincander family are cursed to disappear forever on the eve of their twelfth birthday.
Never recovering from the loss of her cousin and the loss of her own future, Beth became a numb and apathetic child, daydreaming away from reality. Now, Beth is to turn 12, and despite her parents’ attempts at saving her, the Wincander witch snatches her in the night.
The daydreaming coping mechanism is interesting. I’d be interested in knowing what her daydreams are about. How does she escape the reality of her situation? I feel that could reveal a lot about your MC.
The witch is Beth’s estranged grandaunt, Serena Wincander, a surly old woman who locks Beth in a cottage where she lives with three ravens. The pets adore Beth and try cheering her up by bringing her cookies, books and games. Still, Beth is homesick and scared.
Here, you might want to ground the reader in a sense of place. As you mention later, it seems the cottage is in a magical world, which I assume is different from the primary world Beth wants to return to. If there is a secondary world, can you paint a picture for me? What is special about it? At the very least, what’s the name (magical world is a bit generic)?
Beth’s estranged grandaunt, Serena Wincander, takes her to a cottage in [insert name of “magical world”], where she meets/encounters/befriends three doting ravens.
But when a very powerful witch comes looking for Beth, Grandaunt Serena hides the girl. For the powerful witch is the one that cursed the Wincanders: the three ravens are, in fact, the stolen Wincander girls, including Beth’s cousin. Now, if Grandaunt Serena fails to hide her too, Beth is next.
Interesting! So, Serena took Beth to spare her from the other witch? I would omit this sentence though: “Now, if Grandaunt Serena fails to hide her too, Beth is next.” (You have already mentioned she hid Beth.)
At discovering her cousin, Beth regains her fiery personality and promises to save her family. She believes the key to breaking the curse lies in the secret history between the two witches but her surly grandaunt refuses to talk about their bad blood.
I think this part is redundant and can be omitted: “At discovering her cousin.”
Personally, I’m not fond of the use of “believes.” By virtue of a character's actions, we will know what he/she believes.
Vowing to save her family, Beth digs into the secret history between the two witches, which may hold the key to breaking the Wincander curse. However, her surly grandaunt refuses to talk about their bad blood.
Beth must discover the truth and break the curse quickly, or she will be turned into a raven and remain trapped in the magical world forever.
Just a minor re-write:
Beth works to uncover the truth before she is turned into a raven and trapped in the magical world forever.
Final Thoughts: I love family-centric stories, and that aspect came through well in this query. As a reader, I don't feel grounded in where this story is taking place, which is a bit of a concern. It seems the Wincanders and the witches live in different worlds, but that wasn't made fully clear to me. Best of luck with getting it published!