Posting a picture of the pink-themed invitiation, Tamara confirmed she's set to welcome a little girl with her husband Jay Rutland.
"'The most beautiful baby shower invites I have ever seen, I have the world's best sister," wrote Tamara – thanking Petra, who has organised the event for her seven-months-pregnant sister.
In the photo, a little jewel-encrusted gold rattle can be seen nestling in a pink box.
Tamara announced her pregnancy in a world exclusive with HELLO! magazine in September, telling readers: "I just feel so lucky".
She was delighted to share news of her "honeymoon baby" just four months after their lavish French Riviera wedding, and just nine months after she met former City stockbroker Jay.
"I never imagined this all happening so quickly. It's crazy how life works in such mysterious ways. Jay loves children and I’m really excited for the next phase of our relationship," she told HELLO! at the time.
And Tamara – whose father is Formula One chief Bernie Ecclestone – says both she and Jay want to make sure the baby is raised with the right values. "I think you can spoil a child with love and attention rather than showering them with materialistic things", she told HELLO!
"My parents instilled strong values in my sister and I, and I want to pass those on to my child. I want to do all the things my mother did: she always put us first and encouraged us to follow our dreams. I want to be there for my child."