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We all know the first line of a query is your "hook." I call the last line the "sinker." You want it to punch them in the face, in a nice, friendly kind of way that makes them unable to forget you after having read the 300 other queries in their inbox.
If you're looking for query advice, but are slightly intimidated by my claws, blade, or just my rolling googly-eyes, check out the query critique boards over at AgentQueryConnect. This is where I got my start, with advice from people smarter than me. Don't be afraid to ask for help with the most critical first step of your writing journey - the query. My comments appear in green.
They were happy—that was all that mattered. Orphaned at a young age and raised by their grandmother Rose, Tommy, Isabelle and Joe have never wanted for anything. Rose had her quirks—she rarely left the house, wore a key around her neck to a locked closet they had never seen her open, and refused to speak about the past—but she loved them, and they were happy together. I like your opening para here. It's got an interesting Gothic feel to it that certainly has me curious. But the last sentence here is a bit unwieldy.
But then a fire sweeps through their house, claiming the lives of Rose and Tommy and blowing the twice-orphaned Isabelle and Joe out of their safe, secluded world. They are sent to France to live with a cousin they've never met, and there they will learn that their grandmother’s secrets go deeper than they ever could have imagined. Same situation - you've got a rather unwieldy sentence here at the end. Also, the proper names of everyone here gets a little confusing, especially with two people dying right at the outset. Find rephrases - just "grandma" etc., to cut down on character name usage.
Briovera is a hauntingly beautiful old chateau set off in the middle of the farmland just outside of Saint-Lô, Normandy, a real town that was dubbed “The Capital of Ruins” after it was bombed into near oblivion during World War II. Again, long sentence here that needs chopped up. Use more economical wording for a query. Its beauty and history are lost on sixteen-year-old Isabelle—she wants nothing more than to leave this strange place as soon as possible. However, thirteen-year-old Joe’s curiosity is immediately piqued by the strange rumors surrounding the estate. Soon after their arrival, a vicious animal begins stalking the farmland, and Isabelle and Joe are forbidden to set foot outside the house on their own. But Joe is determined to uncover the truth at any cost.
And when Isabelle glimpses a boy who looks like Tommy—whose body was never found—she, too, resolves to discover what exactly is out there in the dark wood surrounding the chateau. She enlists the help of Marc, a farmhand who lives and works on the estate. But Marc—and his elusive brother Tristan—have secrets of their own. Again, a lot of name usage here that you don't necessarily need.
Nothing Gold (109,000 words) is a unique YA mystery, infused with history and the stirrings of romance, set in a place where the protagonists’ struggles parallel the real history of a town that had to literally rebuild itself from its ashes. This novel works as a stand-alone, but I envision it as the first in a series.
I currently live in Brooklyn and work in Manhattan as a social media manager for a global cosmetics company. The seed of my story was planted the year after college, when I applied for an English teaching assistantship in France and was placed in the small, seemingly uninteresting town of Saint-Lô. This is good information - you're showing that you are internet savvy when it comes to marketing and social media, as well as illustrating that you know what you're talking about in terms of the historicity of your setting.
Overall this is really intriguing. You've got a great setting and a mysterious storyline - in fact, it might be a little too mysterious. A query isn't meant to tease the agent, but to show them that you've got a real story here. I think you can almost get away with it, but then there's another allusion to mystery with the brothers, without really saying why they need to have secrets in the first place. It's got a bit of a paranormal / Hound of the Baskervilles feel to it as well... which is great, unless it is a paranormal masquerading as a mystery... or is it the other way around?
I'm honestly a little torn on this one because I can see that going into too much explanation is going to make this a weighty query, but you don't want to just tease either. I think the historical Gothic feel and the paranormal touch might be high-concept enough to reel an agent in on its own, but I'm not positive you're illustrating that there's enough of a story here to merit such a high word count. Anything over 100k for a debut is a risk. Try to pare down.