Fifi, 31, uploaded the image to social media site Instagram with the caption: 'My beautiful baby sister... Gone but never forgotten. I love you, Peaches x'.
The mum-of-two died yesterday at the country home she shared with husband Thomas Cohen in Wrotham, Kent.
The weatherbeaten childhood snap shows a young Fifi Trixibelle and Peaches pulling faces as they stand side-by-side in a garden. It comes just a day after Peaches posted a childhood photo to her own Instagram - of herself with mum Paula Yates before Paula's death in 2000.
While Peaches and youngest Geldof sister Pixie were outgoing socialites who sought out careers in showbiz - Peaches as a TV presenter and columnist, and Pixie as a model - Fifi, who works in public relations, has chosen to remain out of the public eye.
Despite their differing personalities, Fifi remained close to her younger sister. The two once shared a flat attached to dad Bob Geldof's home in London.
Pop star Lily Allen, a longterm friend of Peaches and her family, was one of the first to pay tribute to the 25-year-old, tweeting: 'My thoughts are with Peaches' family at this awful time. I hope they get to grieve in peace. Peaches, rest in peace gorgeous girl'.
@lilyallen My thoughts are with Peaches' family at this awful time. I hope they get to grieve in peace. Peaches, rest in peace gorgeous girl
- Lily Allen (@lilyallen) April 7, 2014
She was joined in her condolences by celebrities as diverse as supermodel Naomi Campbell, singer Ellie Goulding, football pundit Gary Lineker and Irish president Michael D. Higgins.
The surprise death has sent shockwaves through the showbiz industry, coming at a time when Peaches had long renounced her party girl lifestyle and seemed to have settled down into domestic bliss with husband Thomas Cohen and their two boys, Astala and Phaedra.