Nina Robertson was Sebastian and Sophie Blake's au pair, who worked for Warren Fox in Spain. In July 2019, Nina took the twins to England with her. Sienna noticed them at the hotel and attempted to go after them but ended up losing them. Sienna went back to the hotel the next day and found the twins playing with their au pair, and befriended Nina. Nina told Sienna about her employer and realised that there is a chance that the man could be Warren. Sienna lied to Nina about her name being "Anna" and that she was looking for an au pair, and offered Nina work. Nina agreed to think about it and get back to her, and decided to meet with her at The Teahouse. Sienna ended up stealing Nina's phone and tried but failed to correctly guess the pass-code. When Nina called her phone, Sienna told her that she must have picked up Nina's phone on accident. When Sienna returned Nina's phone, she managed to steal Nina's passport and purchased a wig from Donovans' Salon to match Nina's hair.
On the day of Maxine Minniver and Damon Kinsella's wedding, which took place at the hotel where Nina and the twins were staying, Sienna ended up taking locking and tying Nina up in the laundry room before taking the twins and attempting to drive to the airport in Nina's car. Nina, Brody Hudson and Liberty Savage caught up to the car, and Nina and Brody convinced Sienna to check Nina's phone for pictures of the twins with their father. Sienna ended up unlocking the door and giving the twins back to Nina, and Liberty convinced her not to get the police involved.
Nina later met with Warren, telling him that she didn't know what Sienna looked like and that she had told her that her name was "Anna". She also told him that she managed to convince Sienna that the twins weren't hers by showing Sienna photos of her brother with the twins. Warren told Nina that he was taking the twins back to Spain and that Sienna would never see them again, also informing Nina that she was fired.