Each person in The Joys of Motherhood has a social role with specific obligations. As a wife, Nnu Ego is obligated to provide for her husband and for children (most importantly for her boys). As senior wife, she is supposed to be a good example to her junior wives, to treat them (and Nnaife) with respect, and not to be jealous. Nnaife, as a husband, is obligated not to neglect any of his wives and to give them children. When his brother dies, he is obligated to assume responsibility for his brother's wives and their children. Nnu Ego's children are obligated to provide for their parents in their old age. Oshia, as the first-born son, is obligated to help pay for his younger siblings' education after he's received his education and has a good job. Nobody in this novel likes their duties very much, and most of the tension occurs when one character fails to fulfill his or her duties. Nnu Ego is the only character who fulfills her duties. However, because Nnaife and her children don't fulfill their duties, her life is not a happy one.
Although Nnu Ego struggles her entire life to fulfill her duty as a good wife and mother, none of the people around her fulfill their duties. Her frustrated life thus has little to do with her inability to adapt to new values and more to do with other people's selfishness.
Even though Oshia was selfish and failed to help his family, his reluctance to support his family when his father is capable of working is justifiable. The problem wasn't Oshia's selfishness, but was Nnaife's.