While I agree with all that has been said, I keep going back to two other uses of the world salt - the first be people who are referred to as "the salt of the earth" and how we talk about someone being "worth his salt".
Salt is required for life and until the invention of canning and freezing, it was the primary means of preservation of food.
"Salt of the earth people" are people who are good and representative of the best attributes of society. I think this would, by and large, describe the families we get to see in this book. They have done their best to live good lives although conflicts with the world and each other are unavoidable.
When we talk about someone who is worth his salt it means that he/she is of a good quality and works hard. Salt was considered so precious that Roman soldiers were paid a "salarium" to buy salt. It is thought this is the origin of the word "salary". Again I feel these are fitting descriptions of the people in the book. The only one who might not be "worth his salt" was Atef who gave up his friend, Mustafa.
Of course you might take my thoughts with a "grain of salt".