This Is Our Undoing by Lorraine Wilson

Could you condemn one child to save another?

In a near-future Europe fracturing under climate change and far-right politics, biologist Lina Stephenson works in the remote Rila Mountains, safely away from London State. When an old enemy dies, Lina's dangerous past resurfaces, putting her family's lives at risk.

Trapped with her vulnerable sister alongside the dead man's family, Lina is facing pressure from all sides: her enemy's eldest son is determined to destroy her in his search for vengeance, whilst his youngest carries a sinister secret...

...But the forest is hiding its own threats and as a catastrophic storm closes in, Lina realises that if she is to save her family, she must become a monster.