In early September 1962 in Modesto, California at the tail end of summer vacation, recent high school graduates and longtime friends, Curt Henderson and Steve Bolander, meet John Milner and Terry "The Toad" Fields at the local Mel's Drive-In parking lot. Despite receiving a $2,000 scholarship from the local Moose lodge, Curt is undecided if he wants to leave the next morning with Steve to go to the northeastern United States to begin college. Steve lets Toad borrow his 1958 Chevrolet Impala for the evening and while he's away at college until Christmas. Steve's girlfriend, Laurie, who also is Curt's sister, is unsure of Steve's leaving, to which he suggests—to Laurie's surprise—they see other people while he is away to "strengthen" their relationship. She is not happy with his proposal.
Curt, Steve, and Laurie go to the local back to school sock hop, while Toad and John begin cruising. En route to the dance, at a stoplight, Curt sees a beautiful blonde girl in a white 1956 Ford Thunderbird. She appears to say "I love you" before disappearing around the corner. After leaving the hop, Curt is desperate to find the mysterious blonde, but is coerced by a group of greasers ("The Pharaohs") to participate in an initiation rite that involves hooking a chain to a police car and ripping out its back axle. Curt is told rumors that "The Blonde" is either a trophy wife or prostitute, which he refuses to believe.
Following a series of arguments, Steve and Laurie split, and John inadvertently picks up Carol, an annoying teenybopper who seems fond of him. Toad, who is normally socially inept with girls, successfully picks up a flirtatious, and somewhat rebellious, girl named Debbie. Meanwhile, Curt learns that the DJ Wolfman Jack broadcasts from just outside Modesto. Inside the radio station, Curt encounters a bearded man he assumes to be the manager. Curt hands the man a message for "The Blonde" to call or meet him. As he walks away, Curt hears the voice of The Wolfman, and, having just seen The Wolfman broadcasting, he realizes he had been speaking with The Wolfman himself.
The other story lines intertwine until Toad and Steve end up on "Paradise Road" to watch John race his yellow deuce coupe against the handsome, but arrogant, Bob Falfa. Earlier, Bob had picked up Laurie, who is now sitting shotgun in his black '55 Chevy. Within seconds of the finish, Bob loses control of his car after blowing a front tire, plunges into a ditch and rolls his car. Steve and John run to the wreck, and a dazed Bob and Laurie stagger out of the car before it explodes. Distraught, Laurie grips Steve tightly and tells him not to leave her. He assures her that he has decided not to leave Modesto after all. The next morning Curt is awakened by the sound of a phone ringing in a telephone booth, which turns out to be "The Blonde". She tells him she might see him cruising tonight, but Curt replies that is not possible, because he will be leaving. At the airfield he says goodbye to his parents, his sister, and friends. As the plane takes off, Curt, gazing out of the window, sees the white Ford Thunderbird belonging to the mysterious blonde.
Prior to the end credits, an on-screen epilogue reveals that John was killed by a drunk driver in December 1964, Toad was reported missing in action near An Lộc in December 1965, Steve is an insurance agent in Modesto, California, and Curt is a writer living in Canada (implying that he may be there as a draft dodger).

Question: How is Steve's girlfriend related to Curt?
Answer: She is Curt's sister.
Alternative answer: She is his sister.