My answer will be brief because I don't have time to review the novel, but I can say confidently (from memory) that the wedding scene has at least two important functions:
1. The scene comes early in the novel and helps to make the reader interested in the fate of this family. The scene is written for the contemporary American reader (not the Lithuanian villager, who wouldn't need the same explanations or descriptions of what happens at the wedding).
2. The wedding scene shows how the family's culture is undergoing changes in the new country. The younger generation doesn't follow all of the customs, such as helping to pay for all the festivities.
The enotes study guide (see the link below) adds a couple of other functions of the wedding scene: "The book starts with hope... At the wedding, the key people in their lives are introduced... The wedding scene also introduces a sense of how strict their budget is and how greatly they fear unemployment."