"She's a bit dismissive of the idea at the moment," Blunt told ET's Kevin Frazier at the premiere of her new film The Huntsman: Winter's War at the Regency Village Theatre in Westwood, California on Monday. "She's quite liking being top dog."
While Blunt admitted that her daughter "has no choice" about becoming a big sister, she does have a strategy that might help ease tensions.
"My plan is just to tell her that this is less about having a sibling and more that she's being promoted," Blunt joked. "She gets to have a worker now that she can boss around."
In her new epic fantasy adventure, Blunt was tasked with wearing a variety of incredibly ornate and intricate costumes, which she said were more difficult to carry around than her growing baby bump.
"With the bump you sort of adjust slowly as you get heavier, but with the dress it's just like someone kind of drops a bear over your back to carry," Blunt explained.
"There was like a huge leather cloak I wore that was made out of braided leather," Blunt recalled. "You know it's heavy when you see four men wrestling it onto the set."
The Huntsman: Winter’s War hits theaters on April 22.
For more on Blunt and Krasinski's upcoming addition to their family, check out the video below.