In the beginning of the third book, House LeGris finds out that Natalia (and other humans) are susceptible to vampire control, if the human drinks vampire blood. This is scene is part of that experiment.
From Protector of the Grey House:
Her nails dug into his flesh as she considered his proposition. Vincent had refrained from drinking her blood for the past month, allowing her to heal and be at full strength for the experiments. She grudgingly accepted his explanation, knowing it was probably the best idea. But now, as Joseph scraped his fangs against her tender skin, she wondered if it had been a mistake. Her nails dug in deeper as he bit her ever so gently on her neck. He retracted his fangs as he bit, knowing better than to draw blood without permission. Instead, he pulled his wrist to her mouth and beckoned her to drink the salty liquid that poured from his wound. She drank more than she meant to as he whispered dark ideas into her ear.
Vincent watched the spectacle, hands forming into fists. Mierka sat at his desk, fascinated by the look on Natalia’s face. Her expression kept changing from a pained expression to one of desire. Mierka guessed the human did not want to enjoy being in Joseph’s arms. When Joseph let the human go, Natalia staggered, trying to catch her balance. She was looking around nervously, as if unsure of those around her. Vincent reached forward and lifted her chin to look in her eyes. The glazed look she had faked with Anthony was threatening to come into her eyes.
“Natalia.” Both were surprised by Joseph’s quiet voice. “Come to me. Your blood is mine tonight.”
Cat Stark 