Sadie is Jude's "perfect" sister they never really got along until season 4. Her name is possibly inspired by the Beatles' song "Sexy Saddie".
When we first met her in season 1, Sadie was jealous of her younger sister's fame. She was shown being mean to Jude and calling her names. At some point in season 1, Jude became jealous when she found out that Sadie was dating her producer, Tommy Quincy. At the end of the season, the girls' family life was shattered when Sadie learned their father was cheating on their mother.
Sadie & Jude grow a little closer but still have a few fights. Sadie began the season still acting spoiled, but this changes when she decides to dump Tommy.
Kwest & Sadie start a relationship but Kwest ends it when Sadie kisses Tommy which also ended Jude's relationship with Tommy. Sadie and Kwest get back together.
Kwest moves into Jude's & Sadie's house. Sadie & Jude are closer. Sadie tries to keep the paparazzi from Jude by saying they have been through a tough time. When Jude says Tommy asked Jude to marry her, Sadie says Tommy & Jude haven't been stable but Jude says he is ready to settle down. When Jude leaves for London Sadie hugs Jude goodbye, looking very sad. Sadie and Kwest broke up.