Must the guardian of a divorced, pregnant woman who has given birth be her father in order to take her back, or is her brother sufficient?

Since you divorced your wife while she was pregnant, she became irrevocably separated from you upon giving birth. You cannot take her back except with a new marriage contract. A guardian (wali) is a condition for the validity of the marriage. The most rightful person to give a woman in marriage is her father. If he is fit for guardianship, then he is the one who should marry her off or appoint someone else. Her brother cannot marry her off unless the father refuses to do so.