Pippa is thought to be throwing her sister a surprise 80s-themed party in London. Where's our invite? Photo: Getty
According to Grazia magazine, Pippa - with a little help from Prince William has had an oil painting commissioned to mark the milestone.
It's widely expected that the Duchess will be walking the red carpet with her husband at the London premiere of Stephen Spielberg's War Horse on Sunday night, with the party taking place afterwards.
Here's hoping Kate doesn't read the press. Otherwise this party won't be much of a surprise...
See our pick of the birthday girl's perfect 10 style moments: