Her birth was before Chain of Memories because, in her introduction into the Orginization all the member were there. Including the members that were destroyed in Chain of Memories. As for Roxas here.
Namine was already with the Orginization in Chain of Memories and, they were born simultaneously so Roxas was born during the incident in Hollow Bastion.
Another Report said:At the same time, Roxas awoke in Twilight Town, and Naminé awoke in Castle Oblivion. Afterwards, the Organization discovered Naminé.
Namine was already with the Orginization in Chain of Memories and, they were born simultaneously so Roxas was born during the incident in Hollow Bastion.