Murder in Masquerade by Mary Winters

Extra, extra, read all about it! Countess turned advice columnist Amelia Amesbury finds herself playing the role of sleuth when a night at the theatre turns deadly.
Victorian Countess Amelia Amesbury's secret hobby, writing an advice column for a London penny paper, has gotten her into hot water before. After all, Amelia will do whatever it takes to help a reader in need. But now, handsome marquis Simon Bainbridge desperately requires her assistance. His beloved younger sister, Marielle, has written Amelia's Lady Agony column seeking advice on her plans to elope with a man her family does not approve of. Determined to save his sister from a scoundrel and the family from scandal, Simon asks Amelia to dissuade Marielle from the ill-advised gambit.
But when the scoundrel makes an untimely exit after a performance of Verdi's Rigoletto, Amelia realizes there's much more at stake than saving a young woman's reputation from ruin. It's going to take more than her letter-writing skills to help the dashing marquis, mend the familial bond, and find the murderer. Luckily, solving problems is her specialty!