Bob Geldof's 31-year-old daughter spoke out after being upset by "the viciousness" of the reaction to the the inquest into her sister's death.
After it was revealed that 25-year-old Peaches was using heroin for three months before her death in April, social media was flooded with comments, including one Twitter user who called her "selfish".
Fifi Geldof has defended late sister Peaches Geldof
Fifi responded via Instagram on Thursday: "You've no idea what you're talking about quite frankly", and advised people to stop sharing their opinions if they deemed Peaches a bad mother.
Fifi was later comforted by a string of sympathetic messages. She told posters: "Thanks for your beautiful words of love and support for myself and Peaches. I appreciate it more than ever today."
The inquest heard that Peaches had been battling drug addiction for two years. Her husband Thomas Cohen told the hearing that he confronted the mother-of-two over her heroin use after finding texts suggesting that she had relapsed.
Fifi Geldof and her sisters in 2005
The weekend before her death he had been staying at his parents' house with both their sons. On Sunday, Thomas' father returned with Phaedra, then 11 months, and spent 30 minutes with Peaches – before leaving his grandson with her.
The TV presenter and columnist was a devoted mother, but struggled to cope with the loss of her own mum Paula Yates when she was 11. Tragically Paula also died of a heroin overdose in 2000.
At this week's inquest, the coroner said: "It has been said this is a case of history repeating itself. But this is not entirely so".
Peaches had tried to stop using drugs, he added. The inquest heard evidence that in 2013 the mother-of-two successfully received counselling and treatment and was working to reduce her dose of methadone, the substitute drug.