What does Marjane's father do about Mehri's relationship with the neighbor Hossein?
In Persepolis, Marjane's father tells the neighbor boy, Hossein, that Mehri, the girl he's been exchanging letters with, is not his daughter, but the maid. As a result of her true identity being exposed, Hossein rejects Mehri completely and gives Marjane's father the letters back.
This incident is an example of one of the major motifs that runs throughout the first half of Persepolis: the treatment of the lower classes in Iran. When Marjane's father returns to the house and hands Marjane the letters, which she has, in fact, been writing for the Mehri, who "like most peasants, she didn't know how to read and write," he explains to her why the love between the two never would have worked out. He simply explains to Marjane that in pre-revolution Iran "you must stay within your own social class."
As a result of this revelation to Marjane, she decides that she wants to do more than just support the revolution from her home. She wants to take to the streets to demand the removal of the Shah, who she sees as the reason Mehri can't marry Hossein.
The rise of the lower classes in Iran is one of the major events that happens after the revolution. This is exemplified in the chapter "The Passport" when Marjane's Uncle Taher suffers a heart attack and needs to leave the country for surgery. The person in charge of the passport is the her uncle and aunt's former window washer who decides to leave the uncle's health to God's will, not surgery in another country.