For the most part, these letters concern Harpo's aching unhappiness because of the fact that the only role model he has for being a husband and a man is that of his indolent father, Albert. Unfortunately, Harpo thinks that he himself is a failure — simply because he can't beat Sofia, like Albert beats Celie. Harpo doesn't realize that his role model is wrong — not Harpo himself — and as long as Harpo is married to Sofia, he will never be able to treat her as Albert treats Celie. Sofia is a strong woman; she won't stand for it.
For that reason, then, Harpo tries unconsciously to be big — big and strong and powerful — like Sofia. But that too fails; all he gets is fat. He becomes the butt of many jokes and gets such a pot belly that he looks pregnant. Yet, in his case, he is not pregnant with anything positive; he is bloated with confusion and resentment and self-pity.
Harpo doesn't realize that he is far luckier than his father. He and Sofia truly love one another — despite their constant fighting. Between Albert and Celie, there is no emotion. Therefore, in a marriage like Harpo and Sofia's, where there is love, there is also room for variation in roles, which is why Harpo and Sofia are able to divide the cooking and the cleaning. In a loveless marriage, such as Albert and Celie have, there is no room for variation. Celie must do what she is told, and Albert does whatever he wants.
Celie tries to explain to Harpo that Albert and Shug are in love and that their relationship works because of their love for one another, but Celie points out that they are not married. A wife and a mistress always have different duties. Harpo still doesn't understand.
Sofia realizes that life with Harpo is a dead-end situation; she needs a vacation. Old indolent Albert, on the other hand, doesn't have to go away to restore his soul because Shug is with him. Because Shug is his mistress and because he loves her, he can allow himself the freedom to "reach over and pick something out her hair." Harpo cannot do this with Sofia because he still thinks that, somehow, he must be dominant and exhibit power and authority.
Harpo is crying as Sofia leaves, wiping his eyes with a baby diaper. Sofia's sisters have rescued her. Harpo has no brothers to rescue him.
Celie, remember, once tried to rescue Nettie when Nettie fled from Fonso and came to live with Celie. But eventually, Nettie had to flee from Albert's lust and brutality. At present, Nettie is only a precious, painful memory to Celie. Neither one of them can rescue the other. In childhood, each had been a haven for the other. No longer.
Sofia, Celie realizes, is lucky to have her sisters. And perhaps that is why Celie gives Sofia the quilt that they made together, the "Sister's Choice" quilt. It is Celie's choice to give it to Sofia. Rarely does Celie have a choice about anything. Once, she planned to give the quilt to Shug if it were perfect, or keep it herself if it were imperfect. At the critical moment of choice, however, neither perfect nor imperfect makes any difference. What makes the difference is that Celie chooses to make Sofia even more of a sister and so, spontaneously, she gives her the quilt out of love and sisterhood.