I found the whole book improbable. I'm not talking about the time travel aspects of it. Or the idea of reincarnation or even fate. I could have gone along with all of that.
What I found unlikely was that the Ravenna/Elizabeth characters were supposed to so different and yet Christian didn't care? The premise was that he was hopelessly in love with Elizabeth that he would do anything to have her. Didn't he mind that "Ravenna" was a completely different person? So are we to believe that he only cared about the package not the content of the person?
I also found it ridiculous that there is a brief mention by Paul of James' love for Ravenna. He talks about how Ravenna is not really James' sister and how he looks longingly at Ravenna. Are you kidding?
We've already established that James loves Sarah beyond all reason. That he risks ostracism for that love. But suddenly Ravenna is so attractive it wipes that love from his mind? Keeping in mind that he brought Sarah on the trip with him and they are apparently doing the hibbity-dibbity all over the ship.
I also found it insulting to genuine love that Christian kept chanting, "I only did it because I loved you." or some such nonsense. He didn't love her. He loved himself. If he'd loved her he would have noticed she seemed different. He would have rescued the man she loved.
Only because I'm so horrifically OCD about finishing books did I finish this one. Had I been a stronger person I would have deleted it and moved on at the first sign of trouble. Perhaps you're stronger than I am.