Well clearly he has a sister, we just saw her. I doubt it will be some random person with the same last name.
If you mean "Was there any previous reference where he had a sister?" I don't remember any, but I haven't read the OotS Origins book.
Considering that his father was a wizard, it is reasonable that a sister of his would go to a school of wizardry (Especially because Roy chose not to learn magic.)
Yeah, I'm pretty sure it's been well established that Roy had a younger sister who went off to magic school like their parents wanted, when Roy insisted on going to Fighter College, then on for his MBA.