One thing I’ve noticed in the game “The Silent Spy” is Nancy’s age when her mother died.  In the original (1930s) books Nancy was about 10 when her mother died.  Then when the books were rewritten she became three.  So if Nancy is 18,  and her mother died 10 years before then she would have been about 8 when Kate died.  So it seems that Her Interactive is going by the original books, or close to it.