In A Thousand Splendid Suns, Mariam's mother Nana hangs herself because she realizes that her daughter has grown fond of her father Jalil. Nana understands that Jalil will not put himself in a position to claim Mariam as his rightful daughter, and she knows that this will break Mariam's heart. She is fearful of their growing relationship and upset by being abandoned, which is one motivating factor for her suicide. Ironically, there might have also been some idea on Nana's part that her death might force Jalil to care for Mariam, and since he could provide some material wealth, a life with him may have been more fulfilling for Mariam.
Later in the novel, Laila does not leave with Tariq and his family to go to Pakistan because she knows that her parents would be heart-broken if she left. The family has already lost two sons in the war, so losing Laila would too would cause too much grief.