She picked up ten pebbles and arranged them vertically, in three columns. […] She put four pebbles in the first column, for Khadija's children, three for Afsoon's, and three in the third column for Nagis's children. Then she added a fourth column. A solitary, eleventh pebble. (1.5.34)
Mariam spends her childhood on the outside looking in. Although she doesn't entirely understand what it means to be a harami, she knows it means that she's isolated from the people that should be her family.
She understood what Nana meant, that a harami was an unwanted thing; that she, Mariam, was an illegitimate person who would never have legitimate claim to the things other people had, things such as love, family, home, acceptance. (1.1.6)
Mariam's status as an illegitimate child leaves her without a true family. Nana holds a great deal of resentment towards Mariam, and Jalil refuses to acknowledge her as a real daughter.
Leaving Afghanistan had been unthinkable to her while Ahmad and Noor were still alive. Now that they were Shaheed, packing up and running was […] a betrayal, a disavowal of the sacrifice her sons had made. (2.21.56)
Again, Mammy's love for her sons is so strong that it actually hurts the people around her. Ironically, her dedication to her family is exactly what puts them in greater danger.
She would never leave her mark on Mammy's heart the way her brothers had, because Mammy's heart was like a pallid beach where Laila's footprints would forever wash away beneath the waves of sorrow that swelled and crashed. (2.20.23)
Mammy has dedicated her life to the memory of her two sons, Ahmad and Noor. While this might seem heartwarming at first, the result is that she's unable to be there for Laila in the way a mother should be.
Her time with Tariq's family always felt natural to Laila, effortless, uncomplicated by […] the personal spites and grudges that infected the air at her own home. (2.18.47)
Now tell us you can't relate to this. It's always nice to escape to a friend's calm home life from your family's craziness, but it's important to remember that no family is perfect.
"It's my father I can't leave," Laila said. "I'm all he has left. His heart couldn't take it either."
Tariq knew this. He knew she could not wipe away the obligations of her life any more than he could his. (2.25.50-51)
Family is hugely important to the characters of A Thousand Splendid Suns. Just like Babi is dedicated to Mammy, Laila is dedicated to Babi. These bonds are the most important things in the characters' lives.
It is strange, Laila thinks, almost unsettling, the thing between Aziza and Tariq. Already Aziza is finishing his sentences and he hers. She hands him things before he asks for them. (4.48.15)
This passage makes the argument that family goes deeper than simply "the people you grew up with." Tariq and Aziza have so many common traits that it's easy to forget that they only met a year earlier.
That night, Zalmai wakes up coughing. Before Laila can move, Tariq swings his legs over the side of the bed. He straps on his prosthesis and walks over to Zalmai, lifts him up into his arms. (4.49.38)
Zalmai is hesitant toward Tariq at first. On some level, he knows that Tariq has replaced Rasheed and that Rasheed isn't coming back. Eventually, though, Tariq proves himself to be a good father to Zalmai by caring for him through thick and thin.
When the children spot Laila, they come running […] Some of them call her Mother. Laila does not correct them. (4.51.29)
Family can be anybody. True, the bonds between Laila and her actual family are some of the strongest in the book, but her deep familial relationships with Mariam and the children of the orphanage show that anyone can fulfill that role.
I regret that I did not make you a daughter to me, that I let you live in that place for all those years. And for what? Fear of losing face? […] How little those things matter to me now after all the loss, all the terrible things I have seen in this cursed war. (4.50.150)
There are few moments in the novel as powerful as this. It's clear from the start that Jalil loves Mariam, but he seems incapable of expressing it out of fear. It's not until he is stripped of the things that he clings to most that he's able to see how important Mariam was to him.