Lost in Times

Derren and Krista are twins, but they're unlike any other siblings you've ever met.
They are Liberi, members of an elite warrior race.
Their sole purpose is to protect reality from the followers of an ancient evil, while hunting for the one being who can ultimately save them all.
The one law they cannot break is time itself. To travel back in time and tamper with the past is forbidden.
The thing is, Derren knows who the savior is. He's seen her face in his dreams every night for years.
So when Derren drags his sister back to WW2 Poland in search of the dream, Krista knows they're in trouble.
Can they find the savior in time, and can they do so without causing a rip in the space/time continuum?