Wearing a silver foil helmet; The Lurkers, by Charles Butler (London: Usborne Ltd., 2006)
Verity can always see the truth. When her hyper-active, super intelligent brother is seduced by aliens who drain his intellect and his enthusiasm for life, she is the only one who starts to see the odd things happening. The aliens want to use her brother as a gateway to the world, and he is keen to be it.
The book ends with them all locked in a chapel with Verity fighting off the Lurkers., She succeeds, but only partially, and like any good horror novel there is an indication that they will be back.
The book works as sf because of Verity's rational response: she notes, records and analyzes. If there is a problem it is that there is slightly too much emphasis on her powerful will, but that is not utterly unusual as an sf conceit (fans are slans!?)