She was released on police bail after her remand order expired.
Police had last Sunday obtained a remand order from the magistrate’s court to hold the siblings, aged 14 and 17, for seven days.
The group, representing friends and family members, says the boys' identities must be kept private and criticises the police's act of putting them in handcuffs.
LS Leonard says he is waiting for instructions from the family.
Samirah Muzaffar's lawyer LS Leonard says she should be released on Thursday.
Activist says the boys' arrest violates the United Nations' code for treatment of minors.
The boys were arrested yesterday afternoon in Taman Tun Dr Ismail over the death of Nazrin Hassan.
Selangor CID chief Fadzil Ahmat says the teenagers were arrested in Taman Tun Dr Ismail at 4pm yesterday.
Selangor police chief Mazlan Mansor says there is no truth to a report that the two teenagers were picked up to help in the probe into their stepfather’s murder.