Would this history with my father even be worth mentioning in the Sponsorship then, as it's not directly related to our relationship?
You could write a very short blurb that your father included you in his application but you ended up returning to Brazil to live with your mother. Make sure to keep documentation of when you moved in together like lease, mail with your names and the date on it (Utilities,banking, etc.) You actually have a longer relationship history and time together in person than many other applicants. Your partner should realize that there are financial implications of sponsoring you that will last for 3 years and that you will need to declare that your are a common law couple to CRA after one year which will have tax implications. You are quite a young couple so a big step.