Captured By the Alien Captain by Nyla Lily

One thing stands between me and my sister’s freedom: Runar, the Medarian king—and captain of the vessel where Flora is being held.

War rages between humans and Medarians, with a deadly poison killing his people. Flora was taken because she’s the key to finding the cure. Every move I make to free her is blocked by Runar. He won’t negotiate, and worse—he refuses to let me out of his sight.
Why? Because, in his eyes, I’m his perfect match. The one made to be at his side. His mate.
He’s wrong. I have to convince him of that, no matter what it takes.
If I’m not careful, I won’t just lose Flora—I’ll lose myself.