Joseph M'Benga's personnel file was glimpsed in a screen graphic in "Under the Cloak of War." It mentions his parents Wangera and Gichinga M'Benga, his brother Nicolas M'Benga, and his sisters Nyawira Ochambo and Sikudhani M'Benga. (No mention of Rukiya, but I guess she's presumed dead.) It's possible that the TOS character could be Nicolas if he's a younger brother, but since he's listed first among the siblings, that implies he's the eldest.
Since Rukiya isn't mentioned in the file, maybe children weren't listed at all, and TOS-M'Benga could be Joseph's son. If TOS-M'Benga was exactly Booker Bradshaw's age, he'd be eight years older than Rukiya, a perfectly reasonable gap between siblings (take it from someone in a family with an unreasonable gap between siblings). That's tight with the birthdate from Joseph's file (he would've had to have become a father at 17), but TOS-M'Benga could easily be a few years younger than his actor (the apparent alternative being, of course, that he's 17 years older than his actor), or he could be Joseph's nephew and sidestep the whole issue.
I don't know if that counts as "Word of God." I take that as meaning something the creators reveal to the general audience about their intentions. A series bible is an internal production document, a tentative guide meant only to suggest possibilities to freelance writers without restricting their creativity. It's not meant as a "public-facing" document, even though Lincoln Enterprises sold the series bibles to interested fans and collectors.
It'd take research to know if it happened in this specific case, but elements of writers' bibles end up being publicized all the time. They're mentioned by actors in interviews, included in press-packets, drawn on in tie-ins. It doesn't seem impossible that Data's original backstory was as widely publicized as the explicit statements that SNW-M'Benga and TOS-M'Benga are the same person and not relatives.
Back to the original point, people change their minds all the time (as in the case of Data), and outright lying is a legitimate publicity tool to preserve dramatic tension. We're all going to be pretty surprised if M'Benga is killed off and then Pike or Number One has some line about how his son just started pre-med at the university of wherever and it must be hard for him to have lost his mother, sister, and now father in such a short period of time.