Mariam was not a happy camper when Rasheed brought Laila home and married her.
Rasheed's entire driving force seemed to be toward fathering a living son. When Mariam was pregnant, she could do no wrong in Rasheed's eyes. After the miscarriage (and all the subsequent miscarriages), Mariam was no longer of any value to Rasheed. Nothing she did could please him.
So, why did it bother Mariam so much to have Laila in the household? Was it the final blow to her ego and self esteem? The final acknowledgement that she could not give Rasheed the only thing he really wanted from her?
Rasheed mostly followed the same pattern of behavior after he married Laila. (Except, he seemed to hop in bed with her a lot sooner, and displayed more interest in the activity.) He was terribly indulgent with Laila, and when he found she was pregnant, she could do no wrong.
He became demanding and impatient when Laila gave birth to a daughter, and didn't want to immediately start working toward having a son. And, after the son was born, he didn't really need the women any more, except as servants.
Mariam warmed up to Laila after Laila tried to defend her from Rasheed's beating. But, was that the real reason? Or did Mariam recognize that Laila's star was falling, just as Mariam's had fallen all those years before?
What is it that drives a man to want to have a son, rather than a daughter?