It wouldn't be too shocking to say that Laila has mommy issues. Her mother, Fariba, pretty much loses her mind when her sons go to war and never quite finds it again. This puts a lot of strain on Mammy and Laila's relationship.
We actually get to see Mammy when she is young and healthy, before Laila is even born. She's one of the faces that Mariam encounters when she first arrives in Kabul and, surprisingly, she is the only sympathetic woman Mariam encounters. Mammy makes such an impact on her that Mariam shows up at Ahmad and Noor's funeral alone, despite the fact that she doesn't leave the house often at all.
The Mammy we see here is vibrant, compassionate, and loving. So what changes?
Family is the most important thing to Mammy—it's so important that the loss of her two sons shakes her to her core. Unable to rationalize the departure, she lashes out against Babi and becomes distant with Laila. Laila knows this and observes that "she would never leave her mark on Mammy's heart the way her brothers had" (2.20.23).
It's not until Laila is almost killed that Mammy is shaken from her stupor and forced to consider leaving Afghanistan for the sake of her family. As always, family is the most important part of her life.