The irony of fashion
Nell is a victim of the capricious nature of human fashion, which during her time of history still outcasted and mistreated some people based on their appearance. As a red-head, she is treated as an outsider without having done anything to merit it. Symbolically, this injustice points backward to a greater systemic injustice, that judgment is often subject to meaningless prejudice and foundation-less bias. It is a reminder of mankind's capricious tendency to believe in opinions that are demonstrably wrong or absurd.
Faux pas
To make matters worse, Nell's life has been fairly difficult emotionally, and her upbringing was less strict than most on account of her emotional sensitivity. Now that has resulted in dramatic irony; Nell struggles to accurately predict and behavior according to social expectations. She is prone to faux pas which redoubles her unique appearance. With these two ironies, she has clearly emerged from the text as a hero figure, because everyone in the community knows exactly who she is.
The archetypal younger sibling
Sometimes, siblings can be rather competitive, and often, siblings rivalries drive people to different competencies. That results in a fairly ironic self-esteem puzzle for Nell. As she goes through life, collecting a lot of negative attention or stand-offish attention, she must also watch in jealousy as her blonde younger sister navigates life perfectly. Dolly only does that by watching Nell and learning from her mistakes, so the real solution to the riddle is that Nell stands out because she lost her mother. She could have benefitted from a mother the way Dolly benefits from her.
Feminism and marriage
Marriage is a nice part of life, one might say. Especially a nice marriage. But Nell and Dolly don't have time or energy to care about whether marriage is pleasant; they only have time to think about how they can get married, because ironically, the community's shared point of view on gender leaves Nell and Dolly without enfranchisement in the society. A woman without a husband is as good as homeless in many cases, and therefore, Nell and Dolly's story can be seen as an allegory for why marriage cannot be the only way women can enfranchise themselves; women deserve equal societal worth as men.
Dolly's conspiracy
The only thing more ironic than the fact that Dolly (for no reason or advantage) chose to betray her sister in the most intimate, existentially cruel way is the fact that Nell forgives her for the mistake. Nell then turns her attention to her own sin and attempts to find peace with God after a long life of frustration and feelings of mistake, shame, embarrassment, and self-defeat. Dolly is the person who made Nell most insecure, and Dolly betrayed her for nothing, like Cain kills Abel.
