SOLVED. Romantic fantasy about a thief training as a librarian by day and cross dressing as her brother by night. Has to solve a crime with I think a guard captain. [s]

[Ead]

This book was likely published in the last five years and I think more recent than that.
As for plot details both siblings are raised by a Duke although I think the title is an unofficial one and he's in charge of all thieves in the city.
The sister poisons the guard on a couple of occasions and has to then heal him as a trainee Royal librarian.
The guard captain finds out that the same person towards the end of the book.
The sister is covering for her brother who is missing and not even the Duke knows he's gone.
This would be written by female author and be in e-book (although may exist in other formats).

[Bryn]

Deception looks likely -- "As the daughter of the most powerful gangster in all of Adalin, Tiana Di Rossi is living a double life. By day, she works as an apprentice librarian in The Glitter--the wealthiest part of the city. By night, she's the leader of the notorious Red League. Tiana's burning the candle at both ends, which comes naturally to the daughter of a fire wraith. While secretly posing as her missing twin brother Killian, her obsidian eyes oversee her father's rowdy gang of rogues as they establish a black market for magically-modified weapons. "

[Ead]

That's the one, thank you so much