We Call Them Witches by India-Rose Bower

Britian, two years after they came . . .
Most people have been devoured by the eldritch creatures.
Sara and her family have been fighting for survival.
And the only thing that works against these monsters - these Witches - is their knowledge of folklore and pagan rituals.
When a girl named Parsley suddenly appears in the garden of their current camp, she cannot remember where she came from or why she's there.
As Sara and Parsley begin to develop feelings for each other, disaster strikes, and Sara's younger brother Noah is snatched by the Witches.
They must find him and set out across desolate moors full of merciless terror, to hunt the very creatures they've been hiding from.
Can they succeed in saving Noah? Will Parsley and Sara's burgeoning love survive these bloodthirsty Witches? and, who can Sara trust in a world void of humanity?