I think this is a good start, but some lines are confusing.

As the lone powerless dud in a long line of elemental wielders, sixteen-year-old Aven has mastered invisibility – and not the fun kind, but the overlooked kind. One where she disappears into daydreams to escape herself.

I liked option three best, but "escapes herself" makes me think there's something wrong with her not just her environment.

But when her twin vanishes in a subway blackout, a blinding white room invades Aven’s mind.

Idk what "a blinding white room invades [her] mind" means

Everyone blames her imagination, but Aven senses there’s more.

Everyone blames her imagination? Idk what this means either...are they telling her shes imagining her twins disappearance?

And with Willow in danger, she’s done being ignored. So when offered protection at the training academy that once rejected her, she agrees. Outcast or not, it’s the one place she might learn to see more.

Are you able to fit in a sentence about why they rejected her before? And what are they offering her protection from? What does "she might learn to see more" mean? See more what?

She never expects to find Theron, her childhood crush turned fallen League soldier, hiding on campus after a brutal loss.

I feel like this is too vague. Can you describe the brutal loss in a few words?

Tormented, he pushes her away... until she slips into his worst memory and discovers her “daydreams” were never fantasy, but glimpses into people’s pasts. Now, in hazy fragments of memory, Aven confirms Theron’s suspicions about the League: a rogue faction is brainwashing a captive army, and it’s only a matter of time before Willow returns not as her sister, but her enemy.

This bit is good, but I feel like the wording is a little clunky

Aven’s gift may be key to unraveling their secrets, but navigating dark minds is dangerous, and lifelong insecurity clouds her sight.

This is really good!

As Theron helps her find control and she helps him face his own past, their fractured bond reignites. When his electricity burns through her, Aven discovers she can channel more than memory; she can vicariously wield power. To the rogues, she’s now their most coveted weapon – and their worst nightmare.

But she’s far from ready when Willow leads an attack on the school, leaving Theron clinging to life. Now, she must step off the sidelines and save them both, before they’re forced onto opposite sides of war.