He had gone away to college and hadn't returned. On a baseball scholarship, he had gone even further into the MLB, a big star these days. Meanwhile, she was still at home, 17 now, when their parents were killed. She has to go live with him, the only relative capable of taking care of her now. They were close growing up, but they were almost ten years apart, and their relationship strained over those formidable years.
Now, while living together, something is changing between them. Animal like, and highly taboo, but it's building nonetheless.
Pairing could be brother/sister or step/brother/sister. If interested in the general plot, PM me and we can discuss. Up for further ideas if interested.