Fifi, whose 25-year-old sister died in April of a heroin overdose, posted a comment on her Instagram page on Thursday, citing her anger at what she called "the viciousness" of the reaction to Peaches' death and the findings of an inquest this week.
Another site user accused Peaches, the daughter of Bob Geldof and the late Paula Yates, of having been "selfish to use drugs in front of them (the children)".
Fifi Geldof hit back at the poster, saying they had "no idea what you're talking about quite frankly".
Later the 31-year-old thanked her followers who had posted messages of support.
"Thanks for your beautiful words of love and support for myself and Peaches. I appreciate it more than ever today," she said.
Peaches Geldof's one-year-old son Phaedra had been in the family home when the former model and journalist died.
An inquest on Tuesday heard that police found 6.9g of "importation quality" heroin stashed in a black cloth bag inside a cupboard over a bedroom door at the house, worth between £350 and £550.
They also discovered a syringe containing residue of heroin inside a sweet box next to the bed, and other drug paraphernalia including burnt spoons, syringes and knotted tights throughout the property.
North West Kent Coroner Roger Hatch said Ms Geldof's death had been "drugs-related" and heroin had played a part.