The supermodel has invited Jude Law- who recently rekindled his romance with the 'Factory Girl' star - to next month's lavish bash, but warned him his girlfriend is not welcome.
A source said: "Since getting back together with Jude, Sienna was looking forward to spending Easter with his family and friends.
"Kate always throws a huge Easter party, which includes an Easter egg hunt during the day, followed by a party in the evening just for adults.
"This year, Kate is having an extravagant 'Alice in Wonderland'- themed night. But she has told Jude that he can't bring Sienna to the bash."
Friends of Kate - who is a close pal of Sienna's ex-boyfriend Rhys Ifans - including her boyfriend Jamie Hince and Jude's ex-wife Sadie Frost have tried to convince her to change her mind, but she is refusing to reverse her decision.
The source added to the Daily Star newspaper: "Jamie, Jude and Sadie have all asked Kate to invite Sienna but she won't budge, saying she is not welcome and that's final.
"Kate's a very loyal friend and doesn't want Rhys to have to spend time with a girl who broke his heart."
Following Sienna's split with Rhys in June 2008, Kate was alleged to have made sure she was "frozen out" by their mutual friends.
When the actress first started dating the 'Notting Hill' actor in August 2007, she was said to have received a warning from the supermodel, who told her: "If you hurt Rhys, you'll have made an enemy for life."