Disclaimer: Naruto is not mine.
Neji had felt a hundred different things for his cousin since she had been born, but he had never felt lust. Not until now, anyway.
When she had been born, he had loved her unconditionally. She had been a pleasant baby, quiet and cuddly and adorable. She had grown a mop of black hair early on and Neji had longed to run his fingers through the hair. He always imagined that it was as soft as the down on a newborn duck, or a puppy's fur. He never touched her though, because he knew it would have been wrong. His father or his uncle would have struck him if he had so much as gotten too close to the baby, undoubtedly, and Neji didn't know whose punishment he feared more.
Then she had turned three and he had been branded by the seal, and before he had even been able to catch his breath his father had been ripped away from him. Neji had struggled with his father's death, but in the end he had been unable to accept it or Hiashi's involvement in it. The love that he had once felt for his cousin faded away and his heart became black and lined with jagged, raw cracks.
He hated her so fiercely and for so long that eventually he forgot how to feel anything else. His hatred consumed him. It flooded his veins with something worse than blood. It was a poison so potent that he found himself being driven by something that was neither alive nor good: spite.
He had almost killed his cousin out of nothing more than foolish spite. He could have crippled her in a thousand different ways, none of which were fatal. In the end he had chosen to aim for her vital organs, with the secret hope of enacting his long-festering desire for revenge. He had wanted to kill her.
He had stared into the depths of the abyss. He had seen what he was in the process of becoming, and he had not backed away. He had faced it with the full knowledge that he was slowly being poisoned, just as surely as he was growing more powerful. And he would have embraced that way of the ninja fully if Naruto hadn't stopped him first.
Thinking back on it later, Neji could appreciate the irony of the situation. It had taken the class runt one-upping him to force him to reexamine the standards by which he lived his life. It had taken Naruto beating him in the Chunin exam and his uncle finally telling him the truth about his father for Neji to finally take an objective look at his feelings for his cousin and to realize how frivolous they were.
Shortly after the Chuunin exam, Neji realized that he loved Hinata once again. But it wasn't the innocent, siblingesque love of their childhood. He did not want to touch her hair to see if it really was as soft as he imagined it would be.
Instead, he was filled with the urge to run his hands over her bare skin, beneath her clothes, across the most private parts of her body. He wanted to taste her skin, to feel her heat, he press his body against hers. He wanted to hear her scream, to see her pant beneath his touch, to see her squirm as he painted pictures of impossibilities on her stomach and breasts.
To his surprise, Neji found himself lusting after his cousin -- and he liked it.