The film is not yet listed on her (Storm Reid's) IMDB nor IMDBPro pages, but the photo above (and below in full), as well as the caption that accompanied it ("Storm Reid as "Emily" and Adepero Oduye as "Eliza" on set of Twelve Years a Slave directed by Steve McQueen") most certainly are.
And with that, we can confirm what I think we all already knew – that Adepero Oduye is playing Eliza in the film; although we didn't know that Storm Reid is playing her daughter Emily.
Next to Patsey (who will be played by Lupita Nyong'o), Eliza is the other prominent female role in the story, and should provide Adepero with enough of a challenge, and lots of screen time to shine.
In short, Eliza is a slave with 2 children – Randall and Emily; we know her as a smart, strong woman (as Solomon Northup describes her), eventually devastated by the loss of her children. As I recall, she has 2 or 3 really of what I'd call powerful, emotional scenes that I think won't leave a single eye dry in the audience when the film is eventually released.
I'd even go as far as to say that it's the kind of role that, despite the fact that it's not THE starring role, could draw lots of acclaim, and awards, depending on how Steve McQueen and Adepero approach the material. Although, really, one could say that about each of the few starring roles in the film.
Regardless, this could be a strong showcase for Adepero and her abilities.
As for Storm Reid, based on her short IMDB page (there are just 2 projects listed on it), this will most certainly be her highest profile role, and she too will get her moment to shine in Twelve Years A Slave.
I did some googling, and found 2 videos of Storm on YouTube, auditioning for parts in Vipaka (the upcoming Forest Whitaker, Anthony Mackie, Sanaa Lathan thriller), and Tyler Perry's Good Deeds (she didn't get either part). Both videos are embedded below, underneath the full photo of she and Adepero on the Twelve Years A Slave Set: