I believe that this question is asking about how Sarah's learning to ride a horse and drive a wagon helps to develop an existing theme rather than develop a brand new theme. Present throughout the book is a theme about the importance of the family unit. That theme is also shown to support the idea that the family unit is most complete when two parents are present in a household. Jacob has been without a wife since the birth of Caleb, and he longs for a wife. The kids also long for a mother. Their hope is that Sarah can fill that roll, yet they all fear that her longing for home and the sea will trump what they bring to the table.
Maggie suggests that Sarah learn to drive the wagon in order to come and visit her. Jacob agrees to do this despite fears that Sarah might decide to ride right out of their lives. Jacob teaches her in hopes that it might help make Sarah more comfortable in that environment. He also hopes that it helps her develop an additional bond to the family and the community. Sarah's learning this new skill might also relate to a theme about loneliness or isolation. Maggie's suggestion shows that she understands how lonely the area can be, and she recognizes the need for human companionship beyond what may be found within a family. Jacob most likely recognizes Sarah's need to be social beyond the walls of his home, so he teaches her to drive the wagon.