I listened to an interview with Jim that was mostly just writer-talk and his history (linked below). It did have some lore questions and someone asked about why Justine didn't mess with the island in Cold Days. His answer was "She wasn't possessed by Nemesis them". It was fascinating and out of left field. In Battle Ground, when Harry asked how long she had been Nemfected, she said "Mortal time is such a limited concept. A few years. Ever since she became close to Lara". That heavily implied to me that she was Nemfected all the way back in White Night because that's when she became Lara's assistant. With this WOJ, her statement almost certainly means an emotional closeness. Maybe Lara found out about Thomas and Justine resuming their physical relationship and it led to Justine and Lara becoming sisterly? I don't know but that's my best guess. We better get answers in the next book though.

Interview: https://www.youtube.com/live/NaAjmJWxnBo?si=t4GTFBqNV_KzInp4