A Samoan Police spokesman told the Herald they could not comment about a second nurse who, along with her colleague, was removed from the medical centre after the deaths for safety reasons.
The spokesman confirmed the senior nurse remains in police custody.
Meanwhile, members of the Nurses Association of Samoa are questioning the decision to charge the senior nurse when post-mortem results have not yet been received from an Australian specialist who carried out the children's examinations.
President for the group, Solialofi Papāli'i, told the Samoa Observer: "There is one other nurse who administered the other vaccination and yet she is not charged, so we are puzzled.
"We were told that the nurse that is now in custody was the one who mixed the vaccination, but we are not sure whether she was the one who mixed both vaccination for both toddlers.''
Following the deaths, all MMR vaccinations in Samoa were stopped until the completion of the investigation. All vaccines - unused or partly used - were also recalled, nationwide.
A coronial inquest into the deaths of the toddlers will be held on September 12.