a. Tom, who played the role of the protagonist in the movie, was the real-life stepbrother of Jim, who played the villain.
b. Tom, who played the role of the protagonist in the movie, was a real-life stepbrother of Jim, who played the villain.
Does (a) necessarily imply that Jim had only one real-life stepbrother?
Does (b) necessarily imply that Jim had more than one real-life stepbrothers?
I think if one were consistent, one would conclude on stepbrother from (a). (b) would be a little more open to interpretation (maybe the speaker doesn't know how many stepbrothers he had).
But I think people aren't really consistent when it comes to these structures.
Many thanks.