Well, first of all, there is no date set for the iPhone yet, and September is just a rumor. The only real date we have is "fall", so it could very well be out in october. Your provider may also bump up your upgrade date a month if you ask nicely. Then there's always the option of forcing your sister to pay the difference.
Really? Did I miss the announcement from Apple that said a new iPhone is coming out this fall?
Some people need to get it into their thick heads that the next iPhone might not come out until next year.