Character Analysis
Oh, yeah: Wang Lung has some other kids, but they're just girls, so they don't matter.
Second Daughter
O-lan kills this daughter as soon as she is born. She is born right in the middle of the famine, so there was no way she was going to make it through the trek South. O-lan kills her to spare her suffering, to make the trip easier, or both.
When Wang Lung meets his second daughter, she is already dead, "[…] Her eyes were closed and the color of her flesh was the color of ashes and her bones stuck up under the skin—a poor silent face that lay there, having endured to the utmost, and there was nothing he could say" (9.40). He buries her and moves on. They never talk about the second daughter again; they even call their third daughter the second one.
Third Daughter (called the Second Daughter)
This daughter is only in Wang Lung's house for a short while. She's engaged when she's only 10 to the son of Liu the merchant.
Even though she is very pretty, a cloud hangs over her, just like her twin brother. The third daughter's unhappiness comes from her mother binding her feet. Wang Lung notices her sadness one day, he asks her about it: "‘Now why have you wept?' […] 'Because my mother binds a cloth about my feet more tightly every day and I cannot sleep at night […] mother said I was not to weep aloud because you are too kind and weak for pain and you might say to leave me as I am, and then my husband would not love me even as you do not love her'" (25.26).
The third daughter makes up for all of O-lan's ugliness. She's prettier than all of the children combined, but her beauty and bound feet make her a target, so she's married off.