Guardians from the Sky - Book 1 of the Acropolis series

In the year 2090, humanity made contact with something in deep space. In 2091, second contact was made, and the invasion of the Chitters began.

Planet Earth is on the brink. Without usable fossil fuels, every world government has completely collapsed, globalization is a thing of the past, and those who remain are huddled in small communities across the barren landscape. Now with the added danger of the Chitters, few are able to keep their heads up in this dark time.
Orson's Island is one of the only communities that have been able to hold back the alien invasion. However, now the Chitters have a new weapon: the deadly and powerful guardians, gigantic creatures that can devastate entire cities in minutes. Everett Allers and his sister Sophie, determined to fight back against the apocalypse, team up with Nikki, a mysterious biohacker, and the Night Banshees, a crew of bomber pilots who have been pivotal in this bleak war. Their only hope is to seek out the Acropolis - a near-mythical place located somewhere in the darkness of space. It is there where humanity will find its salvation - if Everett and his friends can make it there.