What effect does Wright create with his opening scene: a family (mother, daughter, two sons) in one room, especially with the women dressing? How does he add or enhance the effect by adding the rat?

In the first scene the playwright is providing imagery to supply a picture and introduce the family. They live in a one bedroom apartment that isn't fit for a "rat." Thus, the rat is significant for the audience or reader in that it allows us to envision the conditions the family lives in. The women dress in the room and do it without shyness because it's normal for them to do so; they don't know any differently.