Cerulean Bond by Catherine Lievens

Palmer moved to the Eiloren clan palace to learn about dragons and to help close the distance between their two species. As the only human doctor there, every day is a test that he can’t afford to fail.

Especially not with Pearl watching his every move.
Killian’s half-sister has been vocal about her hatred for humans, and when Killian’s siblings start getting sick even as Palmer treats them, he’s afraid she’ll be proven right.
Rune has spent his life trying not to get the attention of his father and yearning for a relationship with his siblings. When Killian asks him to work with Palmer, he sees a chance to prove his loyalty to his brother. He doesn’t expect to fall for the wary, determined human who refuses to be protected like fragile glass, but he does.

As Palmer fights to save his patients and his reputation, he discovers their illness isn’t natural—it’s poison, and all signs point to Pearl. But gathering evidence that she’s involved means painting a target on his own back—and Pearl’s already preparing her next strike.