Cadence is an unreliable narrator because she literally cannot remember the "accident." The novel explores how trauma fragments memory. The Spanish phrase lagunas mentales (mental gaps) perfectly describes her journey.

Family of Liars: An In-depth Analysis of E. Lockhart's Prequel

It is important to note that E. Lockhart wrote a prequel titled Family of Liars (2022). The Spanish version of the prequel is titled Una familia de mentirosos: El regreso (or similar depending on the publisher).