Hassan is one of six children. Other than Mehtab, we don't really get to know the others that well—they're kind of just a bunch of kids making noise as the family moves from one place to another. Here's a general outline so we can keep them straight, and at least know who's older than whom:
Umar is the oldest child. Hassan goes on adventures into London with him as a teenager, and he stays in Lumière as an adult, where he owns two local garages and has four boys.
Mukhar is one of Hassan's younger brothers; he ends up working as a software designer in Helsinki, Finland.
Arash is Hassan's other younger brother. He becomes a law professor at Columbia University in New York City.
Zainab is Hassan's youngest sister, and the baby of the family. She's kind of a mini-Papa: very assertive and very much a go-getter. For example, she's the one who tells Papa to wave the white flag when Mallory is outside freezing for days in the Haji's driveway. When she grows up, she marries the son of Uday Joshi, the restaurateur who was Papa's great nemesis in Mumbai, and together they own a successful chain of restaurants in Mumbai.