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.
I’m pleased to offer my YA novel RED LIGHTNING for your consideration. I honestly think it's better to jump in with the hook. It's purely opinion, but I always put title, word count, etc at the bottom. The agent can probably assume that you're pleased to offer it :) Livia is hands off and walls up when it comes to boys. Nice! I like the hook - get it out there first and don't bury it under an opening line that doesn't really convey anything important. She’s determined to maintain the safety she earned when she sent her abusive father to prison for life. Then news of her cousin’s disappearance catapults her
Livia realizes that her family has lied to her about what really happened that day. Hmm... how? Following leads, she hacks into a Roman History reenactment website and discovers a secret society Err... the secret society isn't terribly secret if they have an internet site, hacking or not with four houses (each with a paterfamilias), a senate, and a military force called the legionnaires. Each house has supernatural abilities that need to remain secret in order for their society to stay safe.
Livia needs to convince this society that her family is not a threat to their way of life and that means joining them. Does it really? If they think she's a threat why would they possibly let her in? Livia has to put her reservations aside to marry a man !?!?!? influential enough to counter the madman who has rediscovered her existence, Wait, so he forgot she existed? And what about the cousin? and convinced a faction that his delusions are fact.
RED LIGHTNING is complete at 120,000 words and available upon request. I have a BA in Latin and History teaching from Brigham Young University.
Word count is long in the tooth, even for a fantasy, or urban fantasy. You're going to want to get his down to under 100k as a debut author. Also, in the opening para you describe it simply as a YA novel which doesn't fly. This is a genre title, and you'll have to label it as such.
The big thing here with me isn't necessarily the query or how it's written, but the bigger questions of plot and motivation. The cousin fades away. The mention of supernatural abilities is kind of on the fly (Does she have them? Why do the societies think she is a threat?) And quite honestly, a teenage girl who was formerly "hands off and walls up" about boys "putting aside reservations" to marry a man (um, how old is he?) in order to protect herself is simply not going to fly with an agent, editor, or with readers.
Get the answers to all the questions I ask above into your query, and pare down that word count. Otherwise I think the premise is quite interesting. We just need to know more about it, and how it's actually impacting Livia and her cousin.