Sister Time is a novel by John Ringo

John Ringo

John Ringo is an American science fiction and military fiction author. He has had several New York Times best sellers. His books range from straightforward science fiction to a mix of military and political thrillers...

 and Julie Cochrane, and is part
of
the Legacy of the Aldenata

Legacy of the Aldenata

The Legacy of the Aldenata, also known as the Posleen War Series is the fictional universe of one of John Ringo's military science fiction series.- Premise :...

series, specifically a spin off that features Michael O'Neal's
daughters Cally and Michelle. Michelle has been raised off planet by the Indowy race, and has been trained in highly advanced mental techniques. As established in the earlier Cally's War

Cally's War

Cally's War is a novel by John Ringo and Julie Cochrane, and is partof the Legacy of the Aldenata series. It follows Michael O'Neal's daughter Cally, who, raised from an early age by her paternal grandfather to be familiar with weapons and tactics, becomes a professional assassin....

, Cally is a dangerous assassin. They meet for the first time since the war, but it is not certain if they will be allies or enemies.