Royal Vengeance by Aimée Carter

The media believes they have uncovered all the royal scandals about the “killer princess,” but there is one game-changing secret remaining that will take the crown in the final book of the Royal Blood series, which Jennifer Lynn Barnes calls “a darker Princess Diaries.”

Evangeline Bright has only one shot to prove she isn’t a traitor: by finding the real mastermind behind the plot that almost killed her father, the King of England.
With the help of Kit, her steadfast boyfriend, Evan must find a way to outsmart the real threat to the throne no matter the cost, because the deadliest enemy the monarchy has faced in over a century is back with a vicious vengeance. And Evan soon uncovers the reason why.
A scandal so huge that if it ever becomes public, it will tear the monarchy apart.
To protect her loved ones, Evan must silence her enemies for good—because if the truth gets out, it won’t just shatter the monarchy. It’ll shatter Evan’s entire family.