Mehtab is your typical bossy sister. She's younger than Hassan and since we hear more about her than the other siblings, she's a bit closer to him than the others. Hassan says that she "is really the only woman I have ever maintained a relationship with over time" (.434). That's important right there. When Hassan moves to Paris, she joins him and helps him manage his home and restaurant.
To be honest, we don't hear much about Mehtab's personal hopes and dreams. When we hear her speak, she's mostly bossing people around or nagging Hassan. In this way, she kind of acts like a fill-in mother figure for Hassan to make sure that he tows the line. She may not achieve a glamorous end, however she is extremely practical and knows how to make things work.