Fifi Geldof, whose 25-year-old sister died in April of a heroin overdose, posted a comment on her Instagram page yesterday expressing 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 Geldof, the daughter of Bob Geldof and the late Paula Yates, of having been selfish to use drugs in front of her children.
Fifi Geldof hit back at the poster, saying "You've no idea what you're talking about quite frankly".
Later the 31-year-old thanked her followers who had posted messages of support, writing: "Thanks for your beautiful words of love and support for myself and Peaches."
Peaches Geldof's one-year-old son Phaedra was in the family home in Wrotham, Kent when the former model and journalist died.
An inquest on Tuesday heard that police found 6.9g of "importation quality" heroin worth between £350 and £550 stashed in a black cloth bag inside a cupboard over a bedroom door.
North West Kent Coroner Roger Hatch said Ms Geldof's death had been "drugs-related" and heroin had played a part.