Is Saeed's marriage to Aunt Fatima unlawful, given that he was previously married to the daughter of her former husband, Ahmed, from another wife, and is this girl considered Saeed's stepdaughter in his marriage to the aunt?

There is no objection to Saeed marrying his maternal aunt, Fatima. His previous marriage to the daughter of her former husband, Ahmed, does not affect this, because this girl is not his aunt's daughter, so she does not prevent the marriage to her mother. This girl is not considered Saeed's stepdaughter; rather, she is a stranger to him. A stepdaughter is the daughter of one's wife, and she becomes forbidden to the stepfather if he has consummated the marriage with her mother.