A Cinderella Story

Samantha "Sam" Montgomery lives in the San Fernando Valley area of Los Angeles, with her widowed father Hal, who runs a popular sports-themed diner. Hal soon marries an exceedingly vain, and selfish woman named Fiona, who has socially-awkward fraternal twin daughters, Brianna and Gabriella. During the 1994 Northridge earthquake, Hal is killed when he runs to save Fiona. Having supposedly left no will, Fiona receives all of his belongings, including the house, the diner, and to her dismay, Sam.
Eight years later, Sam is employed as a waitress at the diner to save money to attend Princeton University, but she is regularly tormented by her stepfamily. Sam struggles to cope socially at North Valley High School, where queen bee and cheerleader Shelby Cummings, also torments her and calls her "Diner Girl" along with other members of the popular clique.
Sam confides in her online pen pal "Nomad" about her dream to attend Princeton, a dream which he also shares. However, Nomad's true identity is Austin Ames, the popular, yet unhappy, quarterback of the school's football team, and Shelby's ex-boyfriend, although she refuses to accept that he has broken up with her. Nomad proposes that they meet in person at the school's Halloween themed homecoming dance. On the night of the dance, Fiona orders Sam to work the night shift at the diner, then leaves to drive Brianna and Gabriella to the dance. Initially reluctant, Sam is convinced by her best friend, Carter Ferrell, to go to the dance and meet her mysterious online friend.
Rhonda, Sam's other best friend, and the rest of the diner's staff also convince her to disobey Fiona and go to the dance anyway. Sam, wearing a mask and a beautiful white dress, meets "Nomad" at the dance, and is surprised to learn that he is Austin, who had become smitten with her upon her entrance. The two decide to leave the party to walk alone and get to know each other a little better. While sharing a romantic dance, Sam and Austin begin to fall in love. But just as Austin is about to unmask her, Sam's cell phone alarm goes off, warning her to return to the diner before Fiona returns at midnight. She leaves without revealing her identity to Austin, and drops her phone on her way out.
Austin picks up her phone and begins a desperate search to figure out who his "Cinderella" really is, but every girl at the dance claims to be the mysterious owner of the phone. Sam is reluctant to reveal her identity to Austin, feeling that he will not accept her due to her being an outsider. When Austin comes into the diner one day, Sam is forced to help him and, after a talk, she attempts to reveal her identity to him, but is cut off by Fiona. Sam's stepsisters discover Sam and Austin's email relationship, and (after having failed to convince Austin that one of them is the owner of the phone) convince Shelby that Sam tried to steal Austin from her. During a pep rally, Shelby, Sam's stepsisters, and the other cheerleaders humiliate her in front of the entire school and expose her identity as well as naming her an impostor. Austin, hurt by Sam's secrecy, does not step up to defend her, and Sam leaves the pep rally feeling humiliated and devastated.
Like Austin, Sam had been accepted to Princeton, only to be duped by Fiona into believing she was rejected. Sam then decides to give up her dreams and resigns herself to working at the diner, but Rhonda gives Sam a pep talk not to lose hope. When her stepsisters come in, they slam the door, causing a guitar to fall off the wall, tearing the wallpaper down with it. Sam then sees her father's words, "Never let the fear of striking out keep you from playing the game," and regains her confidence (in real life this quote came from Babe Ruth). She stands up to Fiona, quits her job at the diner, and moves in with Rhonda, who also quits along with the entire diner staff, all of whom had only put up with so many years of Fiona's abuse after Hal's death for Sam's sake. The customers, who witness the entire scene, promptly leave in disgust as well.
Before the school's homecoming football game, Sam confronts Austin about his cowardice and lies. Before the final play of the game, he sees Sam making her way out of the stands, and finally stands up to his father, saying he wants to attend Princeton rather than simply play football all his life. He chases after Sam and apologizes. She accepts his apology and they share their first kiss as rain falls over the drought-plagued valley. Soon after, Sam finds Hal's will hidden in her childhood fairy-tale book, stating that all of his money &amp; possessions actually belong to her. Since this leaves her as their rightful and legal owner, Sam sells her stepfamily's fancy cars so that she can pay for college, and Fiona, who signed the will as a witness but claims to have never seen it before, is arrested by the police.
Fiona, Brianna, and Gabriella are made by the District Attorney to work off the money they stole from Sam at the diner, which is restored to its former glory by its new owners, Sam and Rhonda. Sam finds that she was in fact accepted by Princeton; the acceptance letter is retrieved from the garbage by her stepsisters. Also, Austin's father comes to accept his son's desire to attend Princeton. Things even work out in the end for Carter as he makes a commercial for acne medication. Shelby, having previously turned Carter down after he defended her at the Halloween dance, aims to pursue him since he is now popular, but after finally seeing her true colors, he turns her down for Astrid, the high school's goth DJ and announcer. The film ends with Sam and Austin, now officially a couple, driving off to Princeton together.

Question: Who discover Sam and Austin's email relationship?
Answer: Sam's stepsisters