Who is Fiona Cleary (Fee) in "The Thorn Birds" most like in "The Kite Runner" and "A Thousand Splendid Suns"?

I would argue that Fiona (Fee) Cleary is most like Baba in The Kite Runner and Nana in A Thousand Splendid Suns. 
At a glance, Baba seems to be a strange choice, but he and Fee share a few similarities. Both are head-strong and resourceful individuals. Both also live under the burden of a terrible secret for many years. Like Baba, Fee favors one of her children above the others. Neither Fee nor Baba fully comprehend the damage their preferences cause until years later. While Baba dotes on Hassan, Fee only has eyes for Frank. In Baba's eyes, Hassan is the son he would have dearly loved to acknowledge, if he could. After all, Hassan is brave, loyal, and conscientious. To Baba, Hassan is everything his other son, Amir, is not. 
In The Kite Runner, we learn that Hassan is the product of an affair between Sanaubar (Ali's wife) and Baba. Amir does not discover his relation to Hassan until a short while before Baba's death, and he is devastated by the revelation. In The Thorn Birds, Fee keeps her own terrible secret from her daughter, Meggie. After Fee becomes separated from her son, Frank, she reveals to Meggie her terrible secret: Fee had always favored Frank because he was the son of her lover, Pakeha, an influential politician. Because Pakeha was married during his affair with Fee, he never acknowledged Frank as his son. Fee confesses to Meggie that her father paid Paddy Cleary to marry her and to give Frank his name.
Fee warns Meggie that both of them are guilty of stealing another woman's husband and that they will pay for their sins. In Fee's eyes, the Church is the feminine equivalent of Pakeha's wife, and Meggie is just as guilty of causing a man to break his vows as she is. Fee laments that she lost her son "in the worst way a mother could" and that she will never be able to see him again.
Similarly, Baba loses Hassan when his past catches up to him; jealous of Baba's attentions to Hassan and feeling guilty about his own mistreatment of his step-brother, Amir accuses Hassan of stealing from him. This results in Hassan and Ali leaving Baba's employ and home. Baba pleads for Hassan and Ali to stay but is unable to detain them. He cannot risk openly acknowledging Hassan as his son because it will destroy his standing in society. Despite great care, both Baba and Fee are deprived of their favorite children. Both are deeply religious individuals who find themselves constrained by the social norms of their communities.
Baba doesn't harbor bitterness about his fate, but Fee does. In this, she is most like Nana in A Thousand Splendid Suns. At this point, it is worth noting that Nana, Baba, and Fee all had affairs that led to great misery in their lives. Nana's lover is Jalil, an extremely wealthy and influential man. He is also her employer. Jalil cannot acknowledge the child he and Nana conceived together, so Mariam (the product of their love) must live as a harami (or illegitimate child).
After Nana becomes pregnant with Mariam, Jalil throws her out of the house. Even Nana's own father disowns and abandons her. Because of her father and Jalil's lack of decency, Nana becomes extremely bitter and distrustful of all men. For his part, Jalil builds Nana and Mariam a small shack or kolba to live in. Despite his actions, Nana never forgives Jalil for his refusal to acknowledge her as his wife. Nana eventually commits suicide and dies a bitter and miserable woman. Both Nana and Fee refuse to accept the bitter consequences of their actions; they believe deep down in their hearts that life has played cruel tricks on them.