i mean nayantara was wearing nagin white costume and after that she never wore that
so it could be possible that it was whole flashback or backtrack after that
as how she was wearing nagin costume if she is not the one and after that she never wore that costume
if there was a fashion show of nagin
it is just to tell audienece that both r manyata daughter and brinda is elder one so she shapeshifted first and both manyatara will know very later about brinda real identity till then both will fight