His to Claim by Evey Embers

Lilith MacBeth never saw much hope for her future. With her family, and the entire population of Earth for that matter, held under the control of Alien invaders from the distant planet Ceres… why would she ever expect much from life?
But all that was soon to change, when Lilith discovers that her sister, Eve, has enrolled herself in an online mail order bride service for Cerean bachelors… Lilith confronts Eve about this haphazard plan, only to be told that Eve has contracted a contagious virus and would never pass the critical inspection points. So, rather than have the scheme fall to pieces and put her family in even more dismal living conditions than they already are, Lilith decides to take the place of Eve!
Soon, she is being shipped off to marry a Martian from Ceres – one she has never met, and knows little about. She’s heard the rumors about these virile alien men, and their strict, passionate ways… When she finally meets her mate, Lilith finds him a foreboding figure with a strictness never before experienced or seen. But, Lilith is willing to take the risk and see if she can handle all he’s got to offer her inexperienced body…