What is the significance of this being the last story in the book in which Marin is mentioned?

This is similar to your last question: The house is Esperanza's reality..... not the dream she envisioned for herself and her family. Nor, is it the house she dreams of for her future. The house on Mango Street, however, served to shape Esperanza into the woman she became.... the neighbors, the environment, and the friends she made stayed with her for the rest of her life. The house on Mango Street is a part of Esperanza herself. We see this significance in the last part of the book.