Ms Rigg-David said her brother suffered from mental illness
The family of a 40-year-old man, who died in custody after being arrested on suspicion of assault, said they are seeking answers from police.
Sean Rigg was held on suspicion of assaulting a police officer and public order offences near Atkins Road, Brixton, south London, on Thursday.
He fell ill and died 90 minutes later. The Independent Police Complaints Commission is looking into the matter.
His sister Samantha Rigg-David said his death has "shocked" the family.
Police have not formally identified him and are still to reveal the results of his post-mortem examination.
Speaking to BBC London, Ms Rigg-David said her brother from Balham, south London, had mental health concerns but was otherwise healthy.
Relatives of Mr Rigg left flowers outside Brixton police station.
"He has a mental illness but he was very fit. From the time of arrest to the time being brought into police custody he's collapsed and died. We've not been told why.
"We are being upright as a family and we have come down here ourselves to find answers.
"All of us are shocked, numb, totally gutted. We are anxious, some of us are very angry," she added.