Both the major plots of Anna Karenina – Anna's affair and Levin's discovery of love – hinge on family life. For Levin, the family is the basic unit of all productive society. His observations of peasants, like Ivan Parmenov, show him that people are most likely to work efficiently and productively when their labor benefits their families directly. He uses this knowledge to influence his relationship with Kitty, whose steady spiritual model teaches Levin how to be a better man. By contrast, Anna's abandonment of her husband, Karenin, and, especially, of her son, Seryozha, destroys her ability to interact productively with society. What is more, once she gives up her family life, she damages her ability to love both Vronsky and her daughter, Annie. Ironically, it is Anna's sacrifice of her family to be with Vronsky that ruins the family feeling she needs in order to be happy with Vronsky.
Vronsky pursues Anna in part because he has no desire for family life; an adulterous love puts a normal family life out of the question.
Anna's fate as an adulterer is so different from her brother's only because she is a woman and society holds her to a different standard.