Arsenic for Tea by Robin Stevens

Schoolgirl detectives Daisy Wells and Hazel Wong are at Daisy's home, Fallingford, for the holidays. Daisy's glamorous mother is throwing a tea party for Daisy's birthday, and the whole family is invited, from eccentric Aunt Saskia to dashing Uncle Felix. But it soon becomes clear that this party isn't really about Daisy at all. Naturally, Daisy is furious.

Then one of their party falls seriously, mysteriously ill—and everything points to poison.

With wild storms preventing anyone from leaving, or the police from arriving, Fallingford suddenly feels like a very dangerous place to be. Not a single person present is what they seem—and everyone has a secret or two. And when someone very close to Daisy looks suspicious, the Detective Society must do everything they can to reveal the truth... no matter the consequences.