Yes, Stepbrother! by Annie Valentine

After a long stint in the Marine’s, Brandon is granted some time off to enjoy his family. With an entire three month summer ahead of him, he is ready to finally be at home.
When he comes home though he is surprised to see that his stepsister Kylie is no longer the annoying little girl he grew up with, but a beautiful, smart, and passionate young woman who is focused on saving the world.

As the two spend more time with one another, they begin a risky relationship that leads them to some of the best moments of their life. However, bitterness starts to creep in when they realize that their love must be a secret and it also must end, for Brandon must go back to Germany in August.
The only problem is, Kylie, in her passion, strayed from her responsible side, and forgot to take her birth control…