Dawn knew all along that he didn't belong to her sister. She knew that sooner or later Buffy's slayer appeal would fade and the darkness clouding his vision would pass. She also happened to be correct in all that she knew.
Dawn knew that Buffy wouldn't send her off with anyone but him. Collecting potentials had turned out to be a less than safe mission, and Buffy trusted him more than anyone, even after his mysterious disappearance and return to their ranks. Danw knew btter than to ask him questions about what happened. He didn't talk about it, and so they lived thier lives as if it had never happened, as if the crater that once was Sunnydale was not his doing.
Dawn knew that hse was in a very delicate position right now, feeling his breath hot on her face as she lay inches from him. She knew that things could go either way after tonight. He would either remember his love for her sister, and obsession that was imprinted in his brain like the chip that had fermented there for years, or he would realize what Dawn already knew: he was meant to be more than her protector.
Dawn knew that her sister still felt that residual obsession as well, but she didn't care much. It felt good to know that she could steal something from the slayer. Buffy was powerless before her, which, though she would never have admitted it out loud, was something she had wanted for years.
Dawn knew he would be hesitant at first, and he had been. After all, it was quite the risk he was taking, becoming involved with the slayer's little, not to mention barely legal, sister. As a matter of fact, it had been on her eighteenth birthday when he first came to her like she knew he would. Maybe he found security in knowing that she was officially an adult. Maybe it was their arrival in Venezuela, the humidity of the air invading his soul.
Dawn knew that what they had was more real than what he and Buffy had once had. He wuold actually love her, actually feel something more than a shadow of an emotion when he was near her. Dawn knew that she was mere inches away from everything she had ever wanted.
And Dawn knew exactly how to reach out and take it.