Starbound Orphans

The aliens didn’t come to destroy Earth. They came for its children.
When alien ships descend over Earth, thousands of young people are taken. Those who survive the first selection are sent aboard the Tower, an enormous alien vessel where every level brings another test—and failure can mean death.
Ivo, Penelope, Elias, and Silas enter the trials as strangers.
Penelope is determined to protect the people she has left. Elias is fighting to survive with a heart that was failing him long before the aliens arrived. Silas is younger than the others, frightened and clever, with secrets even he doesn’t understand.
And Ivo doesn’t remember who he is at all.
Found injured beneath the ruins of an escape pod, Ivo has no memory of his real name or where he came from. But as the four fight their way through the Tower, fragments of another life begin returning—an unfamiliar world, a royal symbol, and a past someone went to extraordinary lengths to erase.
The trials were supposed to turn them against one another.
Instead, they become a family.
And for Ivo and Penelope, the bond forming between them may eventually become something neither expected—if they live long enough to discover what it means.
Because the Tower is hiding more than deadly tests. There are prisoners aboard who were never meant to be found. A resistance on Earth knows secrets about the alien Empire. And buried somewhere in its records is a mysterious designation:
A-17.
The deeper Ivo searches for his past, the more dangerous the answers become.
Because the Empire isn’t only taking children from Earth.
It may be searching for one.

They were strangers when the aliens took them. Surviving the Tower will make them a family.