Maviarab,
Chillout1984
You're both right in your own way.However, I can't escape the feeling that there is something deeper behind Nadia's feelings for Kane (even if it is just sincere sisterly love). Perhaps Nadia's childhood was not as happy as it may seem, perhaps Cynthia was too strict with her and Nadia was left with a complex of a child without a mother's care.
Maybe Nadia couldn't find in Stefan the support and the person she could trust and who would really love her (as it was between Olivia and Kane in their common childhood). And maybe that's why Nadia is trying so desperately and diligently to fix everything and to get back the person she was deprived of since childhood.
Of course, these are just my feelings and I could be wrong too, but there's clearly something behind her obsession with helping Kane. And Nadia doesn't seem like someone who's doing all this just because Cynthia or Callista told her to. Yes, we definitely need a deep and thorough heart-to-heart talk between Nadia and Kane. I may not like her sadistic nature, but she deserves to have Kane listen to her.