Sepheres was born to Victoria of House Lancaster. It's no surprise that he, like one of his elder brothers, looks nothing like his father. In fact, he looks so little like Titus that no one was believed Sepheres to be the head-of-house's son. As a young child, he was not permitted to know his real father.
The boy was raised a son of House Lancaster, but he was never, ever permitted to feel like he belonged. His eldest brother and father both made sure of that. Despite being raised, technically, as a son of Lancaster, he was no better than a slave. And some may have argued that he would have been better off being a slave instead.
Atenra tried, and his cousin tried. His uncle, even. But none of them were really able to absorb the brunt of what happened.
Eventually Sepheres would be sold into service at a brothel, sold as the slave he was. It's not so bad there - Helene is a good person, and she tries to make it as easy as possible. It's a decent place, and he knows that things could be much, much worse for him.