The 29-year-old singer - who got engaged to DJ Tom Crane over the festive season while holidaying in the Maldives - is planning a Tropicana-themed cocktail party at London nightspot Mahiki and though her Girls Aloud bandmates Cheryl Cole, Nicola Roberts and Kimberley Walsh are all invited to celebrate with the couple, the Los Angeles-based Irish beauty will not be on the guest list.
A friend of the couple said: "The proposal was romantic and very private so they haven't had a chance to celebrate with all of their friends yet.
"There has been so much animosity flying around Girls Aloud in recent months, and Sarah and Tom just want this night to be about them. Most of the band has been invited though - Cheryl, Kimberley and Nicola. Just not Nadine.
"But they knew Nadine would be in LA anyway, so it's not entirely personal."
Sarah is also said to have banned Tom, 29, from DJ-ing at their party as she wants him to be able to enjoy himself.
The source added to the Daily Mirror newspaper: "Tom wanted to DJ but Sarah's banned him from the booth.
"She said she doesn't him working on their big night. She really put her foot down about it."
Nadine - who got engaged to boyfriend Jason Bell in September last year - recently admitted she hadn't spoken to her bandmates in months.
Her spokesperson admitted they had no idea if the 'Insatiable'singer had been invited to Sarah's party.
They said: "She's not here, she's in LA at the moment. I don't know if she's been invited."